diff --git a/modules/entity/spec/sorted_state_adapter.spec.ts b/modules/entity/spec/sorted_state_adapter.spec.ts
index e88c12adc4..cc9116b6b0 100644
--- a/modules/entity/spec/sorted_state_adapter.spec.ts
+++ b/modules/entity/spec/sorted_state_adapter.spec.ts
@@ -1,4 +1,4 @@
-import { EntityStateAdapter, EntityState } from '../src/models';
+import { EntityStateAdapter, EntityState, Update } from '../src/models';
 import { createEntityAdapter } from '../src/create_adapter';
 import {
   BookModel,
@@ -277,4 +277,72 @@ describe('Sorted State Adapter', () => {
       },
     });
   });
+
+  it('should let you add one entity to the state with upsert()', () => {
+    const withOneEntity = adapter.upsertOne(
+      {
+        id: TheGreatGatsby.id,
+        changes: TheGreatGatsby,
+      },
+      state
+    );
+
+    expect(withOneEntity).toEqual({
+      ids: [TheGreatGatsby.id],
+      entities: {
+        [TheGreatGatsby.id]: TheGreatGatsby,
+      },
+    });
+  });
+
+  it('should let you update an entity in the state with upsert()', () => {
+    const withOne = adapter.addOne(TheGreatGatsby, state);
+    const changes = { title: 'A New Hope' };
+
+    const withUpdates = adapter.upsertOne(
+      {
+        id: TheGreatGatsby.id,
+        changes,
+      },
+      withOne
+    );
+
+    expect(withUpdates).toEqual({
+      ids: [TheGreatGatsby.id],
+      entities: {
+        [TheGreatGatsby.id]: {
+          ...TheGreatGatsby,
+          ...changes,
+        },
+      },
+    });
+  });
+
+  it('should let you upsert many entities in the state', () => {
+    const firstChange = { title: 'Zack' };
+    const secondChange = { title: 'Aaron' };
+    const withMany = adapter.addAll([TheGreatGatsby], state);
+
+    const withUpserts = adapter.upsertMany(
+      [
+        { id: TheGreatGatsby.id, changes: firstChange },
+        { id: AClockworkOrange.id, changes: secondChange },
+      ],
+      withMany
+    );
+
+    expect(withUpserts).toEqual({
+      ids: [AClockworkOrange.id, TheGreatGatsby.id],
+      entities: {
+        [TheGreatGatsby.id]: {
+          ...TheGreatGatsby,
+          ...firstChange,
+        },
+        [AClockworkOrange.id]: {
+          ...AClockworkOrange,
+          ...secondChange,
+        },
+      },
+    });
+  });
 });
diff --git a/modules/entity/spec/unsorted_state_adapter.spec.ts b/modules/entity/spec/unsorted_state_adapter.spec.ts
index 0d94a28b2b..2a2655cc93 100644
--- a/modules/entity/spec/unsorted_state_adapter.spec.ts
+++ b/modules/entity/spec/unsorted_state_adapter.spec.ts
@@ -217,4 +217,72 @@ describe('Unsorted State Adapter', () => {
       },
     });
   });
+
+  it('should let you add one entity to the state with upsert()', () => {
+    const withOneEntity = adapter.upsertOne(
+      {
+        id: TheGreatGatsby.id,
+        changes: TheGreatGatsby,
+      },
+      state
+    );
+
+    expect(withOneEntity).toEqual({
+      ids: [TheGreatGatsby.id],
+      entities: {
+        [TheGreatGatsby.id]: TheGreatGatsby,
+      },
+    });
+  });
+
+  it('should let you update an entity in the state with upsert()', () => {
+    const withOne = adapter.addOne(TheGreatGatsby, state);
+    const changes = { title: 'A New Hope' };
+
+    const withUpdates = adapter.upsertOne(
+      {
+        id: TheGreatGatsby.id,
+        changes,
+      },
+      withOne
+    );
+
+    expect(withUpdates).toEqual({
+      ids: [TheGreatGatsby.id],
+      entities: {
+        [TheGreatGatsby.id]: {
+          ...TheGreatGatsby,
+          ...changes,
+        },
+      },
+    });
+  });
+
+  it('should let you upsert many entities in the state', () => {
+    const firstChange = { title: 'First Change' };
+    const secondChange = { title: 'Second Change' };
+    const withMany = adapter.addAll([TheGreatGatsby], state);
+
+    const withUpserts = adapter.upsertMany(
+      [
+        { id: TheGreatGatsby.id, changes: firstChange },
+        { id: AClockworkOrange.id, changes: secondChange },
+      ],
+      withMany
+    );
+
+    expect(withUpserts).toEqual({
+      ids: [TheGreatGatsby.id, AClockworkOrange.id],
+      entities: {
+        [TheGreatGatsby.id]: {
+          ...TheGreatGatsby,
+          ...firstChange,
+        },
+        [AClockworkOrange.id]: {
+          ...AClockworkOrange,
+          ...secondChange,
+        },
+      },
+    });
+  });
 });
diff --git a/modules/entity/src/models.ts b/modules/entity/src/models.ts
index 416bb6ae30..9f763391b9 100644
--- a/modules/entity/src/models.ts
+++ b/modules/entity/src/models.ts
@@ -63,6 +63,9 @@ export interface EntityStateAdapter<T> {
 
   updateOne<S extends EntityState<T>>(update: Update<T>, state: S): S;
   updateMany<S extends EntityState<T>>(updates: Update<T>[], state: S): S;
+
+  upsertOne<S extends EntityState<T>>(update: Update<T>, state: S): S;
+  upsertMany<S extends EntityState<T>>(updates: Update<T>[], state: S): S;
 }
 
 export type EntitySelectors<T, V> = {
diff --git a/modules/entity/src/sorted_state_adapter.ts b/modules/entity/src/sorted_state_adapter.ts
index 0641234b65..15aaecbad0 100644
--- a/modules/entity/src/sorted_state_adapter.ts
+++ b/modules/entity/src/sorted_state_adapter.ts
@@ -106,6 +106,43 @@ export function createSortedStateAdapter<T>(selectId: any, sort: any): any {
     }
   }
 
+  function upsertOneMutably(update: Update<T>, state: R): DidMutate;
+  function upsertOneMutably(update: any, state: any): DidMutate {
+    return upsertManyMutably([update], state);
+  }
+
+  function upsertManyMutably(updates: Update<T>[], state: R): DidMutate;
+  function upsertManyMutably(updates: any[], state: any): DidMutate {
+    const added: T[] = [];
+    const updated: Update<T>[] = [];
+
+    for (let index in updates) {
+      const update = updates[index];
+      if (update.id in state.entities) {
+        updated.push(update);
+      } else {
+        added.push({
+          ...update.changes,
+          id: update.id,
+        });
+      }
+    }
+
+    const didMutateByUpdated = updateManyMutably(updated, state);
+    const didMutateByAdded = addManyMutably(added, state);
+
+    switch (true) {
+      case didMutateByAdded === DidMutate.None &&
+        didMutateByUpdated === DidMutate.None:
+        return DidMutate.None;
+      case didMutateByAdded === DidMutate.Both ||
+        didMutateByUpdated === DidMutate.Both:
+        return DidMutate.Both;
+      default:
+        return DidMutate.EntitiesOnly;
+    }
+  }
+
   function merge(models: T[], state: R): void;
   function merge(models: any[], state: any): void {
     models.sort(sort);
@@ -147,8 +184,10 @@ export function createSortedStateAdapter<T>(selectId: any, sort: any): any {
     removeAll,
     addOne: createStateOperator(addOneMutably),
     updateOne: createStateOperator(updateOneMutably),
+    upsertOne: createStateOperator(upsertOneMutably),
     addAll: createStateOperator(addAllMutably),
     addMany: createStateOperator(addManyMutably),
     updateMany: createStateOperator(updateManyMutably),
+    upsertMany: createStateOperator(upsertManyMutably),
   };
 }
diff --git a/modules/entity/src/unsorted_state_adapter.ts b/modules/entity/src/unsorted_state_adapter.ts
index dcf1d53214..61ff410bdb 100644
--- a/modules/entity/src/unsorted_state_adapter.ts
+++ b/modules/entity/src/unsorted_state_adapter.ts
@@ -123,6 +123,43 @@ export function createUnsortedStateAdapter<T>(selectId: IdSelector<T>): any {
     return DidMutate.None;
   }
 
+  function upsertOneMutably(update: Update<T>, state: R): DidMutate;
+  function upsertOneMutably(update: any, state: any): DidMutate {
+    return upsertManyMutably([update], state);
+  }
+
+  function upsertManyMutably(updates: Update<T>[], state: R): DidMutate;
+  function upsertManyMutably(updates: any[], state: any): DidMutate {
+    const added: T[] = [];
+    const updated: any[] = [];
+
+    for (let index in updates) {
+      const update = updates[index];
+      if (update.id in state.entities) {
+        updated.push(update);
+      } else {
+        added.push({
+          ...update.changes,
+          id: update.id,
+        });
+      }
+    }
+
+    const didMutateByUpdated = updateManyMutably(updated, state);
+    const didMutateByAdded = addManyMutably(added, state);
+
+    switch (true) {
+      case didMutateByAdded === DidMutate.None &&
+        didMutateByUpdated === DidMutate.None:
+        return DidMutate.None;
+      case didMutateByAdded === DidMutate.Both ||
+        didMutateByUpdated === DidMutate.Both:
+        return DidMutate.Both;
+      default:
+        return DidMutate.EntitiesOnly;
+    }
+  }
+
   return {
     removeAll,
     addOne: createStateOperator(addOneMutably),
@@ -130,6 +167,8 @@ export function createUnsortedStateAdapter<T>(selectId: IdSelector<T>): any {
     addAll: createStateOperator(addAllMutably),
     updateOne: createStateOperator(updateOneMutably),
     updateMany: createStateOperator(updateManyMutably),
+    upsertOne: createStateOperator(upsertOneMutably),
+    upsertMany: createStateOperator(upsertManyMutably),
     removeOne: createStateOperator(removeOneMutably),
     removeMany: createStateOperator(removeManyMutably),
   };