diff --git a/federation-js/CHANGELOG.md b/federation-js/CHANGELOG.md
index dcaf81e89..d3b9dcab5 100644
--- a/federation-js/CHANGELOG.md
+++ b/federation-js/CHANGELOG.md
@@ -4,7 +4,7 @@
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
-- _Nothing yet! Stay tuned._
+- Handle `@external` validation edge case for interface implementors [#4284](https://github.com/apollographql/apollo-server/pull/4284)
 
 ## 0.16.7
 
diff --git a/federation-js/src/composition/validate/postComposition/__tests__/externalUnused.test.ts b/federation-js/src/composition/validate/postComposition/__tests__/externalUnused.test.ts
index ac768b07d..e3156c017 100644
--- a/federation-js/src/composition/validate/postComposition/__tests__/externalUnused.test.ts
+++ b/federation-js/src/composition/validate/postComposition/__tests__/externalUnused.test.ts
@@ -302,4 +302,79 @@ describe('externalUnused', () => {
     const warnings = validateExternalUnused({ schema, serviceList });
     expect(warnings).toEqual([]);
   });
+
+  it('does not error when @external is used on a field of a concrete type that implements a shared field of an implemented interface', () => {
+    const serviceA = {
+      typeDefs: gql`
+        type Car implements Vehicle @key(fields: "id") {
+          id: ID!
+          speed: Int
+        }
+        interface Vehicle {
+          id: ID!
+          speed: Int
+        }
+      `,
+      name: 'serviceA',
+    };
+    const serviceB = {
+      typeDefs: gql`
+        extend type Car implements Vehicle @key(fields: "id") {
+          id: ID! @external
+          speed: Int @external
+        }
+        interface Vehicle {
+          id: ID!
+          speed: Int
+        }
+      `,
+      name: 'serviceB',
+    };
+    const serviceList = [serviceA, serviceB];
+    const { schema } = composeServices(serviceList);
+    const errors = validateExternalUnused({ schema, serviceList });
+    expect(errors).toHaveLength(0);
+  });
+
+  it('does error when @external is used on a field of a concrete type is not shared by its implemented interface', () => {
+    const serviceA = {
+      typeDefs: gql`
+        type Car implements Vehicle @key(fields: "id") {
+          id: ID!
+          speed: Int
+          wheelSize: Int
+        }
+        interface Vehicle {
+          id: ID!
+          speed: Int
+        }
+      `,
+      name: 'serviceA',
+    };
+    const serviceB = {
+      typeDefs: gql`
+        extend type Car implements Vehicle @key(fields: "id") {
+          id: ID! @external
+          speed: Int @external
+          wheelSize: Int @external
+        }
+        interface Vehicle {
+          id: ID!
+          speed: Int
+        }
+      `,
+      name: 'serviceB',
+    };
+    const serviceList = [serviceA, serviceB];
+    const { schema } = composeServices(serviceList);
+    const errors = validateExternalUnused({ schema, serviceList });
+    expect(errors).toMatchInlineSnapshot(`
+      Array [
+        Object {
+          "code": "EXTERNAL_UNUSED",
+          "message": "[serviceB] Car.wheelSize -> is marked as @external but is not used by a @requires, @key, or @provides directive.",
+        },
+      ]
+    `);
+  });
 });
diff --git a/federation-js/src/composition/validate/postComposition/externalUnused.ts b/federation-js/src/composition/validate/postComposition/externalUnused.ts
index 3da582ec6..1cdeddede 100644
--- a/federation-js/src/composition/validate/postComposition/externalUnused.ts
+++ b/federation-js/src/composition/validate/postComposition/externalUnused.ts
@@ -160,6 +160,55 @@ export const externalUnused: PostCompositionValidator = ({ schema }) => {
 
           if (hasMatchingRequiresOnType) continue;
 
+          /**
+           * @external fields can be required when an interface is returned by
+           * a field and its concrete implementations need to be defined in a
+           * service which use non-key fields from other services. Take for example:
+           *
+           *  // Service A
+           *  type Car implements Vehicle @key(fields: "id") {
+           *    id: ID!
+           *    speed: Int
+           *  }
+           *
+           *  interface Vehicle {
+           *    id: ID!
+           *    speed: Int
+           *  }
+           *
+           *  // Service B
+           *  type Query {
+           *    vehicles: [Vehicle]
+           *  }
+           *
+           *  extend type Car implements Vehicle @key(fields: "id") {
+           *    id: ID! @external
+           *    speed: Int @external
+           *  }
+           *
+           *  interface Vehicle {
+           *    id: ID!
+           *    speed: Int
+           *  }
+           *
+           *  Service B defines Car.speed as an external field which is okay
+           *  because it is required for Query.vehicles to exist in the schema
+           */
+          const fieldsOnInterfacesImplementedByParentType: Set<string> = new Set();
+
+          // Loop over the parent's interfaces
+          for (const _interface of parentType.getInterfaces()) {
+            // Collect the field names from each interface in a set
+            for (const fieldName in _interface.getFields()) {
+              fieldsOnInterfacesImplementedByParentType.add(fieldName);
+            }
+          }
+
+          // If the set contains our field's name, no error is generated
+          if (fieldsOnInterfacesImplementedByParentType.has(externalFieldName)) {
+            continue;
+          }
+
           errors.push(
             errorWithCode(
               'EXTERNAL_UNUSED',