diff --git a/lib/ecto/adapter.ex b/lib/ecto/adapter.ex
index a2ed61a943..6cc63ab622 100644
--- a/lib/ecto/adapter.ex
+++ b/lib/ecto/adapter.ex
@@ -60,6 +60,11 @@ defmodule Ecto.Adapter do
   """
   @callback checkout(adapter_meta, config :: Keyword.t(), (() -> result)) :: result when result: var
 
+  @doc """
+  Returns true if a connection has been checked out.
+  """
+  @callback checked_out?(adapter_meta) :: boolean
+
   @doc """
   Returns the loaders for a given type.
 
diff --git a/lib/ecto/repo.ex b/lib/ecto/repo.ex
index 664e41e12a..a9c698b893 100644
--- a/lib/ecto/repo.ex
+++ b/lib/ecto/repo.ex
@@ -227,6 +227,11 @@ defmodule Ecto.Repo do
         adapter.checkout(meta, opts, fun)
       end
 
+      def checked_out? do
+        {adapter, meta} = Ecto.Repo.Registry.lookup(get_dynamic_repo())
+        adapter.checked_out?(meta)
+      end
+
       @compile {:inline, get_dynamic_repo: 0, with_default_options: 2}
 
       def get_dynamic_repo() do
@@ -460,6 +465,28 @@ defmodule Ecto.Repo do
   """
   @callback checkout((() -> result), opts :: Keyword.t()) :: result when result: var
 
+  @doc """
+  Returns true if a connection has been checked out.
+
+  This is true if inside a `c:Ecto.Repo.checkout/2` or
+  `c:Ecto.Repo.transaction/2`.
+
+  ## Examples
+
+      MyRepo.checked_out?
+      #=> false
+
+      MyRepo.transaction(fn ->
+        MyRepo.checked_out? #=> true
+      end)
+
+      MyRepo.checkout(fn ->
+        MyRepo.checked_out? #=> true
+      end)
+
+  """
+  @callback checked_out?() :: boolean
+
   @doc """
   Loads `data` into a struct or a map.
 
diff --git a/lib/ecto/repo/preloader.ex b/lib/ecto/repo/preloader.ex
index 353e89e23c..b9713ae39f 100644
--- a/lib/ecto/repo/preloader.ex
+++ b/lib/ecto/repo/preloader.ex
@@ -121,7 +121,7 @@ defmodule Ecto.Repo.Preloader do
 
   # Then we execute queries in parallel
   defp maybe_pmap(preloaders, repo_name, opts) do
-    if match?([_,_|_], preloaders) and not Ecto.Repo.Transaction.in_transaction?(repo_name) and
+    if match?([_,_|_], preloaders) and not checked_out?(repo_name) and
          Keyword.get(opts, :in_parallel, true) do
       # We pass caller: self() so the ownership pool knows where
       # to fetch the connection from and set the proper timeouts.
@@ -138,6 +138,11 @@ defmodule Ecto.Repo.Preloader do
     end
   end
 
+  defp checked_out?(repo_name) do
+    {adapter, meta} = Ecto.Repo.Registry.lookup(repo_name)
+    adapter.checked_out?(meta)
+  end
+
   # Then we unpack the query results, merge them, and preload recursively
   defp preload_assocs(
          [{assoc, query?, loaded_ids, loaded_structs, preloads} | assocs],
diff --git a/test/ecto/repo_test.exs b/test/ecto/repo_test.exs
index 3675032d67..4de5aa5b5a 100644
--- a/test/ecto/repo_test.exs
+++ b/test/ecto/repo_test.exs
@@ -1362,6 +1362,7 @@ defmodule Ecto.RepoTest do
       def loaders(_, _), do: raise "not implemented"
       def init(_), do: raise "not implemented"
       def checkout(_, _, _), do: raise "not implemented"
+      def checked_out?(_), do: raise "not implemented"
       def ensure_all_started(_, _), do: raise "not implemented"
     end
 
diff --git a/test/mix/tasks/ecto.create_drop_test.exs b/test/mix/tasks/ecto.create_drop_test.exs
index f99a6b02b3..af22d3ef21 100644
--- a/test/mix/tasks/ecto.create_drop_test.exs
+++ b/test/mix/tasks/ecto.create_drop_test.exs
@@ -14,6 +14,7 @@ defmodule Mix.Tasks.Ecto.CreateDropTest do
     def loaders(_, _), do: raise "not implemented"
     def init(_), do: raise "not implemented"
     def checkout(_, _, _), do: raise "not implemented"
+    def checked_out?(_), do: raise "not implemented"
     def ensure_all_started(_, _), do: raise "not implemented"
 
     def storage_up(_), do: Process.get(:storage_up) || raise "no storage_up"
@@ -28,6 +29,7 @@ defmodule Mix.Tasks.Ecto.CreateDropTest do
     def loaders(_, _), do: raise "not implemented"
     def init(_), do: raise "not implemented"
     def checkout(_, _, _), do: raise "not implemented"
+    def checked_out?(_), do: raise "not implemented"
     def ensure_all_started(_, _), do: raise "not implemented"
   end
 
diff --git a/test/support/test_repo.exs b/test/support/test_repo.exs
index 7778447e1c..10ea3a2184 100644
--- a/test/support/test_repo.exs
+++ b/test/support/test_repo.exs
@@ -20,9 +20,19 @@ defmodule Ecto.TestAdapter do
     {:ok, Supervisor.child_spec({Task, fn -> :timer.sleep(:infinity) end}, []), %{meta: :meta}}
   end
 
-  def checkout(_mod, _opts, fun) do
+  def checkout(mod, _opts, fun) do
     send self(), {:checkout, fun}
-    fun.()
+    Process.put({mod, :checked_out?}, true)
+
+    try do
+      fun.()
+    after
+      Process.delete({mod, :checked_out?})
+    end
+  end
+
+  def checked_out?(mod) do
+    Process.get({mod, :checked_out?}) || false
   end
 
   ## Types