diff --git a/integration-tests/aws-sdk/addCustomAttributes.test.ts b/integration-tests/aws-sdk/addCustomAttributes.test.ts
new file mode 100644
index 00000000..19106d03
--- /dev/null
+++ b/integration-tests/aws-sdk/addCustomAttributes.test.ts
@@ -0,0 +1,60 @@
+import { USER_POOL_AWS_DEFAULTS } from "../../src/services/cognitoService";
+import { withCognitoSdk } from "./setup";
+
+describe(
+  "CognitoIdentityServiceProvider.addCustomAttributes",
+  withCognitoSdk((Cognito) => {
+    it("updates a user pool", async () => {
+      const client = Cognito();
+
+      // create the user pool client
+      const up = await client
+        .createUserPool({
+          PoolName: "pool",
+        })
+        .promise();
+
+      const describeResponse = await client
+        .describeUserPool({
+          UserPoolId: up.UserPool?.Id!,
+        })
+        .promise();
+
+      expect(describeResponse.UserPool).toMatchObject({
+        SchemaAttributes: USER_POOL_AWS_DEFAULTS.SchemaAttributes,
+      });
+
+      await client
+        .addCustomAttributes({
+          UserPoolId: up.UserPool?.Id!,
+          CustomAttributes: [
+            {
+              AttributeDataType: "String",
+              Name: "test",
+            },
+          ],
+        })
+        .promise();
+
+      const describeResponseAfterUpdate = await client
+        .describeUserPool({
+          UserPoolId: up.UserPool?.Id!,
+        })
+        .promise();
+
+      expect(describeResponseAfterUpdate.UserPool).toMatchObject({
+        SchemaAttributes: [
+          ...(USER_POOL_AWS_DEFAULTS.SchemaAttributes ?? []),
+          {
+            AttributeDataType: "String",
+            DeveloperOnlyAttribute: false,
+            Mutable: true,
+            Name: "custom:test",
+            Required: false,
+            StringAttributeConstraints: {},
+          },
+        ],
+      });
+    });
+  })
+);
diff --git a/src/__tests__/testDataBuilder.ts b/src/__tests__/testDataBuilder.ts
index 0481d78c..b0a7345c 100644
--- a/src/__tests__/testDataBuilder.ts
+++ b/src/__tests__/testDataBuilder.ts
@@ -1,5 +1,6 @@
 import { v4 } from "uuid";
 import { AppClient } from "../services/appClient";
+import { USER_POOL_AWS_DEFAULTS } from "../services/cognitoService";
 import { Group, User, UserPool } from "../services/userPoolService";
 
 export const id = (prefix: string, number?: number) =>
@@ -84,7 +85,8 @@ export const userPool = (partial?: Partial<UserPool>): UserPool => {
     MfaConfiguration: partial?.MfaConfiguration ?? undefined,
     Name: partial?.Name ?? undefined,
     Policies: partial?.Policies ?? undefined,
-    SchemaAttributes: partial?.SchemaAttributes ?? undefined,
+    SchemaAttributes:
+      partial?.SchemaAttributes ?? USER_POOL_AWS_DEFAULTS.SchemaAttributes,
     SmsAuthenticationMessage: partial?.SmsAuthenticationMessage ?? undefined,
     SmsConfiguration: partial?.SmsConfiguration ?? undefined,
     SmsConfigurationFailure: partial?.SmsConfigurationFailure ?? undefined,
diff --git a/src/targets/addCustomAttributes.test.ts b/src/targets/addCustomAttributes.test.ts
new file mode 100644
index 00000000..8543e2eb
--- /dev/null
+++ b/src/targets/addCustomAttributes.test.ts
@@ -0,0 +1,146 @@
+import { ClockFake } from "../__tests__/clockFake";
+import { newMockCognitoService } from "../__tests__/mockCognitoService";
+import { newMockUserPoolService } from "../__tests__/mockUserPoolService";
+import { TestContext } from "../__tests__/testContext";
+import * as TDB from "../__tests__/testDataBuilder";
+import {
+  GroupNotFoundError,
+  InvalidParameterError,
+  UserNotFoundError,
+} from "../errors";
+import { CognitoService, UserPoolService } from "../services";
+import {
+  AddCustomAttributes,
+  AddCustomAttributesTarget,
+} from "./addCustomAttributes";
+
+const originalDate = new Date();
+
+describe("AddCustomAttributes target", () => {
+  let addCustomAttributes: AddCustomAttributesTarget;
+  let clock: ClockFake;
+  let mockCognitoService: jest.Mocked<CognitoService>;
+
+  beforeEach(() => {
+    clock = new ClockFake(originalDate);
+
+    mockCognitoService = newMockCognitoService();
+    addCustomAttributes = AddCustomAttributes({
+      clock,
+      cognito: mockCognitoService,
+    });
+  });
+
+  it("appends a custom attribute to the user pool", async () => {
+    const userPool = TDB.userPool();
+    const mockUserPoolService = newMockUserPoolService(userPool);
+
+    mockCognitoService.getUserPool.mockResolvedValue(mockUserPoolService);
+
+    const newDate = new Date();
+    clock.advanceTo(newDate);
+
+    await addCustomAttributes(TestContext, {
+      UserPoolId: "test",
+      CustomAttributes: [
+        {
+          AttributeDataType: "String",
+          Name: "test",
+        },
+      ],
+    });
+
+    expect(mockUserPoolService.updateOptions).toHaveBeenCalledWith(
+      TestContext,
+      {
+        ...userPool,
+        SchemaAttributes: [
+          ...(userPool.SchemaAttributes ?? []),
+          {
+            Name: "custom:test",
+            AttributeDataType: "String",
+            DeveloperOnlyAttribute: false,
+            Mutable: true,
+            Required: false,
+            StringAttributeConstraints: {},
+          },
+        ],
+        LastModifiedDate: newDate,
+      }
+    );
+  });
+
+  it("can create a custom attribute with no name", async () => {
+    const userPool = TDB.userPool();
+    const mockUserPoolService = newMockUserPoolService(userPool);
+
+    mockCognitoService.getUserPool.mockResolvedValue(mockUserPoolService);
+
+    const newDate = new Date();
+    clock.advanceTo(newDate);
+
+    await addCustomAttributes(TestContext, {
+      UserPoolId: "test",
+      CustomAttributes: [
+        {
+          AttributeDataType: "String",
+        },
+      ],
+    });
+
+    expect(mockUserPoolService.updateOptions).toHaveBeenCalledWith(
+      TestContext,
+      {
+        ...userPool,
+        SchemaAttributes: [
+          ...(userPool.SchemaAttributes ?? []),
+          {
+            Name: "custom:null",
+            AttributeDataType: "String",
+            DeveloperOnlyAttribute: false,
+            Mutable: true,
+            Required: false,
+            StringAttributeConstraints: {},
+          },
+        ],
+        LastModifiedDate: newDate,
+      }
+    );
+  });
+
+  it("throws if an attribute with the name already exists", async () => {
+    const userPool = TDB.userPool({
+      SchemaAttributes: [
+        {
+          Name: "custom:test",
+          AttributeDataType: "String",
+          DeveloperOnlyAttribute: false,
+          Mutable: true,
+          Required: false,
+          StringAttributeConstraints: {},
+        },
+      ],
+    });
+    const mockUserPoolService = newMockUserPoolService(userPool);
+
+    mockCognitoService.getUserPool.mockResolvedValue(mockUserPoolService);
+
+    await expect(
+      addCustomAttributes(TestContext, {
+        UserPoolId: "test",
+        CustomAttributes: [
+          {
+            AttributeDataType: "String",
+            Name: "test",
+          },
+        ],
+      })
+    ).rejects.toEqual(
+      new InvalidParameterError(
+        "custom:test: Existing attribute already has name custom:test."
+      )
+    );
+
+    expect(mockUserPoolService.updateOptions).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/targets/addCustomAttributes.ts b/src/targets/addCustomAttributes.ts
new file mode 100644
index 00000000..14b7d99d
--- /dev/null
+++ b/src/targets/addCustomAttributes.ts
@@ -0,0 +1,55 @@
+import {
+  AddCustomAttributesRequest,
+  AddCustomAttributesResponse,
+} from "aws-sdk/clients/cognitoidentityserviceprovider";
+import { InvalidParameterError } from "../errors";
+import { Services } from "../services";
+import { assertParameterLength } from "./utils/assertions";
+import { Target } from "./Target";
+
+export type AddCustomAttributesTarget = Target<
+  AddCustomAttributesRequest,
+  AddCustomAttributesResponse
+>;
+
+type AddCustomAttributesServices = Pick<Services, "clock" | "cognito">;
+
+export const AddCustomAttributes =
+  ({
+    clock,
+    cognito,
+  }: AddCustomAttributesServices): AddCustomAttributesTarget =>
+  async (ctx, req) => {
+    assertParameterLength("CustomAttributes", 1, 25, req.CustomAttributes);
+
+    const userPool = await cognito.getUserPool(ctx, req.UserPoolId);
+
+    await userPool.updateOptions(ctx, {
+      ...userPool.options,
+      SchemaAttributes: [
+        ...(userPool.options.SchemaAttributes ?? []),
+        ...req.CustomAttributes.map(({ Name, ...attr }) => {
+          const name = `custom:${Name ?? "null"}`;
+
+          if (userPool.options.SchemaAttributes?.find((x) => x.Name === name)) {
+            throw new InvalidParameterError(
+              `${name}: Existing attribute already has name ${name}.`
+            );
+          }
+
+          return {
+            AttributeDataType: "String",
+            DeveloperOnlyAttribute: false,
+            Mutable: true,
+            Required: false,
+            StringAttributeConstraints: {},
+            Name: name,
+            ...attr,
+          };
+        }),
+      ],
+      LastModifiedDate: clock.get(),
+    });
+
+    return {};
+  };
diff --git a/src/targets/targets.ts b/src/targets/targets.ts
index 4f03e503..3bd432b8 100644
--- a/src/targets/targets.ts
+++ b/src/targets/targets.ts
@@ -1,3 +1,4 @@
+import { AddCustomAttributes } from "./addCustomAttributes";
 import { AdminAddUserToGroup } from "./adminAddUserToGroup";
 import { AdminConfirmSignUp } from "./adminConfirmSignUp";
 import { AdminCreateUser } from "./adminCreateUser";
@@ -46,6 +47,7 @@ import { UpdateUserPoolClient } from "./updateUserPoolClient";
 import { VerifyUserAttribute } from "./verifyUserAttribute";
 
 export const Targets = {
+  AddCustomAttributes,
   AdminAddUserToGroup,
   AdminConfirmSignUp,
   AdminCreateUser,
diff --git a/src/targets/utils/assertions.ts b/src/targets/utils/assertions.ts
new file mode 100644
index 00000000..ca582329
--- /dev/null
+++ b/src/targets/utils/assertions.ts
@@ -0,0 +1,46 @@
+import { InvalidParameterError } from "../../errors";
+
+/**
+ * Assert a required parameter has a value. Throws an InvalidParameterError.
+ *
+ * @param name Name of the parameter to include in the error message
+ * @param parameter Parameter to assert
+ * @param message Custom full message for the error, optional
+ */
+export function assertRequiredParameter<T>(
+  name: string,
+  parameter: T,
+  message?: string
+): asserts parameter is NonNullable<T> {
+  if (!parameter) {
+    throw new InvalidParameterError(
+      message ?? `Missing required parameter ${name}`
+    );
+  }
+}
+
+/**
+ * Asserts an array parameter has a length between min and max. Throws an InvalidParameterError.
+ *
+ * @param name Name of the parameter to include in the error message
+ * @param min Minimum length
+ * @param max Maximum length
+ * @param parameter Parameter to assert
+ */
+export function assertParameterLength<T>(
+  name: string,
+  min: number,
+  max: number,
+  parameter: readonly T[]
+): asserts parameter {
+  if (parameter.length < min) {
+    throw new InvalidParameterError(
+      `Invalid length for parameter ${name}, value: ${parameter.length}, valid min length: ${min}`
+    );
+  }
+  if (parameter.length > max) {
+    throw new InvalidParameterError(
+      `Invalid length for parameter ${name}, value: ${parameter.length}, valid max length: ${max}`
+    );
+  }
+}