diff --git a/backend/src/apiserver/client/BUILD.bazel b/backend/src/apiserver/client/BUILD.bazel
index 2c77ce87295..34f43f794cf 100644
--- a/backend/src/apiserver/client/BUILD.bazel
+++ b/backend/src/apiserver/client/BUILD.bazel
@@ -18,7 +18,6 @@ go_library(
     importpath = "github.com/kubeflow/pipelines/backend/src/apiserver/client",
     visibility = ["//visibility:public"],
     deps = [
-        "//backend/src/apiserver/common:go_default_library",
         "//backend/src/common/util:go_default_library",
         "//backend/src/crd/pkg/client/clientset/versioned:go_default_library",
         "//backend/src/crd/pkg/client/clientset/versioned/typed/scheduledworkflow/v1beta1:go_default_library",
diff --git a/backend/src/apiserver/client/argo.go b/backend/src/apiserver/client/argo.go
index 6fe15530fb2..0fb78360437 100644
--- a/backend/src/apiserver/client/argo.go
+++ b/backend/src/apiserver/client/argo.go
@@ -21,15 +21,10 @@ import (
 	argoprojv1alpha1 "github.com/argoproj/argo/pkg/client/clientset/versioned/typed/workflow/v1alpha1"
 	"github.com/cenkalti/backoff"
 	"github.com/golang/glog"
-	"github.com/kubeflow/pipelines/backend/src/apiserver/common"
 	"github.com/pkg/errors"
 	"k8s.io/client-go/rest"
 )
 
-const (
-	PodNamespace = "POD_NAMESPACE"
-)
-
 type ArgoClientInterface interface {
 	Workflow(namespace string) argoprojv1alpha1.WorkflowInterface
 }
@@ -39,9 +34,6 @@ type ArgoClient struct {
 }
 
 func (argoClient *ArgoClient) Workflow(namespace string) argoprojv1alpha1.WorkflowInterface {
-	if len(namespace) == 0 {
-		namespace = common.GetStringConfig(PodNamespace)
-	}
 	return argoClient.argoProjClient.Workflows(namespace)
 }
 
diff --git a/backend/src/apiserver/client/argo_fake.go b/backend/src/apiserver/client/argo_fake.go
index 597d9715f87..5a2b212984b 100644
--- a/backend/src/apiserver/client/argo_fake.go
+++ b/backend/src/apiserver/client/argo_fake.go
@@ -16,6 +16,7 @@ package client
 
 import (
 	argoprojv1alpha1 "github.com/argoproj/argo/pkg/client/clientset/versioned/typed/workflow/v1alpha1"
+	"github.com/kubeflow/pipelines/backend/src/common/util"
 	"github.com/pkg/errors"
 )
 
@@ -28,6 +29,9 @@ func NewFakeArgoClient() *FakeArgoClient {
 }
 
 func (c *FakeArgoClient) Workflow(namespace string) argoprojv1alpha1.WorkflowInterface {
+	if len(namespace) == 0 {
+		panic(util.NewResourceNotFoundError("Namespace", namespace))
+	}
 	return c.workflowClientFake
 }
 
diff --git a/backend/src/apiserver/client/kubernetes_core_fake.go b/backend/src/apiserver/client/kubernetes_core_fake.go
index 2c40849ebda..1ea06ac5c08 100644
--- a/backend/src/apiserver/client/kubernetes_core_fake.go
+++ b/backend/src/apiserver/client/kubernetes_core_fake.go
@@ -15,6 +15,7 @@
 package client
 
 import (
+	"github.com/kubeflow/pipelines/backend/src/common/util"
 	v1 "k8s.io/client-go/kubernetes/typed/core/v1"
 )
 
@@ -23,6 +24,9 @@ type FakeKuberneteCoreClient struct {
 }
 
 func (c *FakeKuberneteCoreClient) PodClient(namespace string) v1.PodInterface {
+	if len(namespace) == 0 {
+		panic(util.NewResourceNotFoundError("Namespace", namespace))
+	}
 	return c.podClientFake
 }
 
diff --git a/backend/src/apiserver/client_manager.go b/backend/src/apiserver/client_manager.go
index 81402594870..8528947da9a 100644
--- a/backend/src/apiserver/client_manager.go
+++ b/backend/src/apiserver/client_manager.go
@@ -149,7 +149,7 @@ func (c *ClientManager) init() {
 	c.argoClient = client.NewArgoClientOrFatal(common.GetDurationConfig(initConnectionTimeout))
 
 	c.swfClient = client.CreateScheduledWorkflowClientOrFatal(
-		common.GetStringConfig(client.PodNamespace), common.GetDurationConfig(initConnectionTimeout))
+		common.GetPodNamespace(), common.GetDurationConfig(initConnectionTimeout))
 
 	c.k8sCoreClient = client.CreateKubernetesCoreOrFatal(common.GetDurationConfig(initConnectionTimeout))
 
@@ -397,7 +397,7 @@ func backfillExperimentIDToRunTable(db *gorm.DB) (retError error) {
 	}
 
 	_, err = db.CommonDB().Exec(`
-		UPDATE 
+		UPDATE
 			run_details, resource_references
 		SET
 			run_details.ExperimentUUID = resource_references.ReferenceUUID
diff --git a/backend/src/apiserver/common/config.go b/backend/src/apiserver/common/config.go
index 5e7b79d259b..b3078254c8f 100644
--- a/backend/src/apiserver/common/config.go
+++ b/backend/src/apiserver/common/config.go
@@ -23,7 +23,8 @@ import (
 )
 
 const (
-	MultiUserMode     string = "MULTIUSER"
+	MultiUserMode string = "MULTIUSER"
+	PodNamespace  string = "POD_NAMESPACE"
 )
 
 func GetStringConfig(configName string) string {
@@ -69,3 +70,7 @@ func GetDurationConfig(configName string) time.Duration {
 func IsMultiUserMode() bool {
 	return GetBoolConfigWithDefault(MultiUserMode, false)
 }
+
+func GetPodNamespace() string {
+	return GetStringConfig(PodNamespace)
+}
diff --git a/backend/src/apiserver/resource/BUILD.bazel b/backend/src/apiserver/resource/BUILD.bazel
index 40e6ce78148..7bcce3a9a48 100644
--- a/backend/src/apiserver/resource/BUILD.bazel
+++ b/backend/src/apiserver/resource/BUILD.bazel
@@ -53,6 +53,7 @@ go_test(
         "@com_github_argoproj_argo//pkg/apis/workflow/v1alpha1:go_default_library",
         "@com_github_ghodss_yaml//:go_default_library",
         "@com_github_pkg_errors//:go_default_library",
+        "@com_github_spf13_viper//:go_default_library",
         "@com_github_stretchr_testify//assert:go_default_library",
         "@io_bazel_rules_go//proto/wkt:timestamp_go_proto",
         "@io_k8s_apimachinery//pkg/apis/meta/v1:go_default_library",
diff --git a/backend/src/apiserver/resource/resource_manager.go b/backend/src/apiserver/resource/resource_manager.go
index 4457adeb8c6..fa0a963ae78 100644
--- a/backend/src/apiserver/resource/resource_manager.go
+++ b/backend/src/apiserver/resource/resource_manager.go
@@ -304,8 +304,16 @@ func (r *ResourceManager) CreateRun(apiRun *api.Run) (*model.RunDetail, error) {
 		}
 	}
 
+	namespace := common.GetNamespaceFromAPIResourceReferences(apiRun.ResourceReferences)
+	if len(namespace) == 0 {
+		if multiuserMode {
+			return nil, util.NewInvalidInputError("Run should specify namespace")
+		} else {
+			namespace = common.GetPodNamespace()
+		}
+	}
 	// Create argo workflow CRD resource
-	newWorkflow, err := r.getWorkflowClient(common.GetNamespaceFromAPIResourceReferences(apiRun.ResourceReferences)).Create(workflow.Get())
+	newWorkflow, err := r.getWorkflowClient(namespace).Create(workflow.Get())
 	if err != nil {
 		return nil, util.NewInternalServerError(err, "Failed to create a workflow for (%s)", workflow.Name)
 	}
@@ -599,6 +607,9 @@ func (r *ResourceManager) ReportWorkflowResource(workflow *util.Workflow) error
 	}
 	runId := workflow.ObjectMeta.Labels[util.LabelKeyWorkflowRunId]
 	jobId := workflow.ScheduledWorkflowUUIDAsStringOrEmpty()
+	if len(workflow.Namespace) == 0 {
+		return util.NewInvalidInputError("Workflow missing namespace")
+	}
 
 	if workflow.PersistedFinalState() {
 		// If workflow's final state has being persisted, the workflow should be garbage collected.
@@ -992,5 +1003,15 @@ func (r *ResourceManager) GetNamespaceFromRunID(runId string) (string, error) {
 		return "", util.Wrap(err, "Failed to get namespace from run id.")
 	}
 	namespace := model.GetNamespaceFromModelResourceReferences(runDetail.ResourceReferences)
+	if len(namespace) == 0 {
+		if common.IsMultiUserMode() {
+			// All runs should have namespace in multi user mode.
+			return "", errors.New("Invalid db data: run doesn't have a namespace resource reference")
+		} else {
+			// When db model doesn't have namespace stored (e.g. legacy runs), use
+			// pod namespace as default.
+			return common.GetPodNamespace(), nil
+		}
+	}
 	return namespace, nil
 }
diff --git a/backend/src/apiserver/resource/resource_manager_test.go b/backend/src/apiserver/resource/resource_manager_test.go
index ae3cefed628..743aa3530ce 100644
--- a/backend/src/apiserver/resource/resource_manager_test.go
+++ b/backend/src/apiserver/resource/resource_manager_test.go
@@ -30,12 +30,17 @@ import (
 	"github.com/kubeflow/pipelines/backend/src/common/util"
 	swfapi "github.com/kubeflow/pipelines/backend/src/crd/pkg/apis/scheduledworkflow/v1beta1"
 	"github.com/pkg/errors"
+	"github.com/spf13/viper"
 	"github.com/stretchr/testify/assert"
 	"google.golang.org/grpc/codes"
 	v1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/types"
 )
 
+func initEnvVars() {
+	viper.Set(common.PodNamespace, "test-ns")
+}
+
 type FakeBadObjectStore struct{}
 
 func (m *FakeBadObjectStore) AddFile(template []byte, filePath string) error {
@@ -60,13 +65,14 @@ func (m *FakeBadObjectStore) GetFromYamlFile(o interface{}, filePath string) err
 
 var testWorkflow = util.NewWorkflow(&v1alpha1.Workflow{
 	TypeMeta:   v1.TypeMeta{APIVersion: "argoproj.io/v1alpha1", Kind: "Workflow"},
-	ObjectMeta: v1.ObjectMeta{Name: "workflow-name", UID: "workflow1"},
+	ObjectMeta: v1.ObjectMeta{Name: "workflow-name", UID: "workflow1", Namespace: "test-ns"},
 	Spec:       v1alpha1.WorkflowSpec{Arguments: v1alpha1.Arguments{Parameters: []v1alpha1.Parameter{{Name: "param1"}}}},
 	Status:     v1alpha1.WorkflowStatus{Phase: v1alpha1.NodeRunning},
 })
 
 // Util function to create an initial state with pipeline uploaded
 func initWithPipeline(t *testing.T) (*FakeClientManager, *ResourceManager, *model.Pipeline) {
+	initEnvVars()
 	store := NewFakeClientManagerOrFatal(util.NewFakeTimeForEpoch())
 	manager := NewResourceManager(store)
 	p, err := manager.CreatePipeline("p1", "", []byte(testWorkflow.ToStringForStore()))
@@ -75,6 +81,7 @@ func initWithPipeline(t *testing.T) (*FakeClientManager, *ResourceManager, *mode
 }
 
 func initWithExperiment(t *testing.T) (*FakeClientManager, *ResourceManager, *model.Experiment) {
+	initEnvVars()
 	store := NewFakeClientManagerOrFatal(util.NewFakeTimeForEpoch())
 	manager := NewResourceManager(store)
 	experiment := &model.Experiment{Name: "e1"}
@@ -84,6 +91,7 @@ func initWithExperiment(t *testing.T) (*FakeClientManager, *ResourceManager, *mo
 }
 
 func initWithExperimentAndPipeline(t *testing.T) (*FakeClientManager, *ResourceManager, *model.Experiment, *model.Pipeline) {
+	initEnvVars()
 	store := NewFakeClientManagerOrFatal(util.NewFakeTimeForEpoch())
 	manager := NewResourceManager(store)
 	experiment := &model.Experiment{Name: "e1"}
@@ -159,7 +167,8 @@ func initWithOneTimeFailedRun(t *testing.T) (*FakeClientManager, *ResourceManage
 	updatedWorkflow.SetLabels(util.LabelKeyWorkflowRunId, runDetail.UUID)
 	updatedWorkflow.Status.Phase = v1alpha1.NodeFailed
 	updatedWorkflow.Status.Nodes = map[string]v1alpha1.NodeStatus{"node1": {Name: "pod1", Type: v1alpha1.NodeTypePod, Phase: v1alpha1.NodeFailed}}
-	manager.ReportWorkflowResource(updatedWorkflow)
+	err = manager.ReportWorkflowResource(updatedWorkflow)
+	assert.Nil(t, err)
 	return store, manager, runDetail
 }
 
@@ -306,6 +315,7 @@ func TestCreateRun_ThroughPipelineID(t *testing.T) {
 			ExperimentUUID: experiment.UUID,
 			DisplayName:    "run1",
 			Name:           "workflow-name",
+			Namespace:      "test-ns",
 			StorageState:   api.Run_STORAGESTATE_AVAILABLE.String(),
 			CreatedAtInSec: 3,
 			Conditions:     "Running",
@@ -352,6 +362,7 @@ func TestCreateRun_ThroughWorkflowSpec(t *testing.T) {
 			ExperimentUUID: expectedExperimentUUID,
 			DisplayName:    "run1",
 			Name:           "workflow-name",
+			Namespace:      "test-ns",
 			StorageState:   api.Run_STORAGESTATE_AVAILABLE.String(),
 			CreatedAtInSec: 2,
 			Conditions:     "Running",
@@ -436,6 +447,7 @@ func TestCreateRun_ThroughPipelineVersion(t *testing.T) {
 			ExperimentUUID: experiment.UUID,
 			DisplayName:    "run1",
 			Name:           "workflow-name",
+			Namespace:      "test-ns",
 			StorageState:   api.Run_STORAGESTATE_AVAILABLE.String(),
 			CreatedAtInSec: 4,
 			Conditions:     "Running",
@@ -1113,8 +1125,9 @@ func TestReportWorkflowResource_ScheduledWorkflowIDEmpty_Success(t *testing.T) {
 	// report workflow
 	workflow := util.NewWorkflow(&v1alpha1.Workflow{
 		ObjectMeta: v1.ObjectMeta{
-			UID:    types.UID(run.UUID),
-			Labels: map[string]string{util.LabelKeyWorkflowRunId: run.UUID},
+			UID:       types.UID(run.UUID),
+			Labels:    map[string]string{util.LabelKeyWorkflowRunId: run.UUID},
+			Namespace: "test-ns",
 		},
 		Status: v1alpha1.WorkflowStatus{Phase: v1alpha1.NodeRunning},
 	})
@@ -1127,6 +1140,7 @@ func TestReportWorkflowResource_ScheduledWorkflowIDEmpty_Success(t *testing.T) {
 		ExperimentUUID: expectedExperimentUUID,
 		DisplayName:    "run1",
 		Name:           "workflow-name",
+		Namespace:      "test-ns",
 		StorageState:   api.Run_STORAGESTATE_AVAILABLE.String(),
 		CreatedAtInSec: 2,
 		Conditions:     "Running",
@@ -1287,20 +1301,22 @@ func TestReportWorkflowResource_ScheduledWorkflowIDNotEmpty_NoExperiment_Success
 
 func TestReportWorkflowResource_WorkflowCompleted(t *testing.T) {
 	store, manager, run := initWithOneTimeRun(t)
+	namespace := "kubeflow"
 	defer store.Close()
 	// report workflow
 	workflow := util.NewWorkflow(&v1alpha1.Workflow{
 		ObjectMeta: v1.ObjectMeta{
-			Name:   run.Name,
-			UID:    types.UID(run.UUID),
-			Labels: map[string]string{util.LabelKeyWorkflowRunId: run.UUID},
+			Name:      run.Name,
+			Namespace: namespace,
+			UID:       types.UID(run.UUID),
+			Labels:    map[string]string{util.LabelKeyWorkflowRunId: run.UUID},
 		},
 		Status: v1alpha1.WorkflowStatus{Phase: v1alpha1.NodeFailed},
 	})
 	err := manager.ReportWorkflowResource(workflow)
 	assert.Nil(t, err)
 
-	wf, err := store.ArgoClientFake.Workflow("").Get(run.Run.Name, v1.GetOptions{})
+	wf, err := store.ArgoClientFake.Workflow(namespace).Get(run.Run.Name, v1.GetOptions{})
 	assert.Nil(t, err)
 	assert.Equal(t, wf.Labels[util.LabelKeyWorkflowPersistedFinalState], "true")
 }
@@ -1311,9 +1327,10 @@ func TestReportWorkflowResource_WorkflowCompleted_FinalStatePersisted(t *testing
 	// report workflow
 	workflow := util.NewWorkflow(&v1alpha1.Workflow{
 		ObjectMeta: v1.ObjectMeta{
-			Name:   run.Name,
-			UID:    types.UID(run.UUID),
-			Labels: map[string]string{util.LabelKeyWorkflowRunId: run.UUID, util.LabelKeyWorkflowPersistedFinalState: "true"},
+			Name:      run.Name,
+			Namespace: "test-ns",
+			UID:       types.UID(run.UUID),
+			Labels:    map[string]string{util.LabelKeyWorkflowRunId: run.UUID, util.LabelKeyWorkflowPersistedFinalState: "true"},
 		},
 		Status: v1alpha1.WorkflowStatus{Phase: v1alpha1.NodeFailed},
 	})
@@ -1328,9 +1345,10 @@ func TestReportWorkflowResource_WorkflowCompleted_FinalStatePersisted_DeleteFail
 	// report workflow
 	workflow := util.NewWorkflow(&v1alpha1.Workflow{
 		ObjectMeta: v1.ObjectMeta{
-			Name:   run.Name,
-			UID:    types.UID(run.UUID),
-			Labels: map[string]string{util.LabelKeyWorkflowRunId: run.UUID, util.LabelKeyWorkflowPersistedFinalState: "true"},
+			Name:      run.Name,
+			Namespace: "test-ns",
+			UID:       types.UID(run.UUID),
+			Labels:    map[string]string{util.LabelKeyWorkflowRunId: run.UUID, util.LabelKeyWorkflowPersistedFinalState: "true"},
 		},
 		Status: v1alpha1.WorkflowStatus{Phase: v1alpha1.NodeFailed},
 	})
diff --git a/backend/src/apiserver/server/BUILD.bazel b/backend/src/apiserver/server/BUILD.bazel
index 2ea70466472..b053f6cd331 100644
--- a/backend/src/apiserver/server/BUILD.bazel
+++ b/backend/src/apiserver/server/BUILD.bazel
@@ -32,6 +32,7 @@ go_library(
         "@com_github_golang_protobuf//jsonpb:go_default_library_gen",
         "@com_github_pkg_errors//:go_default_library",
         "@com_github_robfig_cron//:go_default_library",
+        "@com_github_spf13_viper//:go_default_library",
         "@com_github_stretchr_testify//assert:go_default_library",
         "@io_bazel_rules_go//proto/wkt:empty_go_proto",
         "@io_bazel_rules_go//proto/wkt:timestamp_go_proto",
diff --git a/backend/src/apiserver/server/test_util.go b/backend/src/apiserver/server/test_util.go
index a130f1e80c8..420baaabc9d 100644
--- a/backend/src/apiserver/server/test_util.go
+++ b/backend/src/apiserver/server/test_util.go
@@ -20,9 +20,11 @@ import (
 	"github.com/argoproj/argo/pkg/apis/workflow/v1alpha1"
 	api "github.com/kubeflow/pipelines/backend/api/go_client"
 	"github.com/kubeflow/pipelines/backend/src/apiserver/client"
+	"github.com/kubeflow/pipelines/backend/src/apiserver/common"
 	"github.com/kubeflow/pipelines/backend/src/apiserver/model"
 	"github.com/kubeflow/pipelines/backend/src/apiserver/resource"
 	"github.com/kubeflow/pipelines/backend/src/common/util"
+	"github.com/spf13/viper"
 	"github.com/stretchr/testify/assert"
 	"google.golang.org/grpc/codes"
 	v1 "k8s.io/apimachinery/pkg/apis/meta/v1"
@@ -65,7 +67,13 @@ var validReferencesOfExperimentAndPipelineVersion = []*api.ResourceReference{
 	},
 }
 
+// This automatically runs before all the tests.
+func initEnvVars() {
+	viper.Set(common.PodNamespace, "test-ns")
+}
+
 func initWithExperiment(t *testing.T) (*resource.FakeClientManager, *resource.ResourceManager, *model.Experiment) {
+	initEnvVars()
 	clientManager := resource.NewFakeClientManagerOrFatal(util.NewFakeTimeForEpoch())
 	resourceManager := resource.NewResourceManager(clientManager)
 	experiment := &model.Experiment{Name: "123"}
@@ -75,6 +83,7 @@ func initWithExperiment(t *testing.T) (*resource.FakeClientManager, *resource.Re
 }
 
 func initWithExperiment_KFAM_Unauthorized(t *testing.T) (*resource.FakeClientManager, *resource.ResourceManager, *model.Experiment) {
+	initEnvVars()
 	clientManager := resource.NewFakeClientManagerOrFatal(util.NewFakeTimeForEpoch())
 	clientManager.KfamClientFake = client.NewFakeKFAMClientUnauthorized()
 	resourceManager := resource.NewResourceManager(clientManager)
@@ -85,6 +94,7 @@ func initWithExperiment_KFAM_Unauthorized(t *testing.T) (*resource.FakeClientMan
 }
 
 func initWithExperimentAndPipelineVersion(t *testing.T) (*resource.FakeClientManager, *resource.ResourceManager, *model.Experiment) {
+	initEnvVars()
 	clientManager := resource.NewFakeClientManagerOrFatal(util.NewFakeTimeForEpoch())
 	resourceManager := resource.NewResourceManager(clientManager)
 
@@ -137,6 +147,7 @@ func initWithOneTimeRun(t *testing.T) (*resource.FakeClientManager, *resource.Re
 
 // Util function to create an initial state with pipeline uploaded
 func initWithPipeline(t *testing.T) (*resource.FakeClientManager, *resource.ResourceManager, *model.Pipeline) {
+	initEnvVars()
 	store := resource.NewFakeClientManagerOrFatal(util.NewFakeTimeForEpoch())
 	manager := resource.NewResourceManager(store)
 	p, err := manager.CreatePipeline("p1", "", []byte(testWorkflow.ToStringForStore()))