diff --git a/src/models/ApiResponse.ts b/src/models/ApiResponse.ts
index eaf7419..20967a4 100644
--- a/src/models/ApiResponse.ts
+++ b/src/models/ApiResponse.ts
@@ -10,6 +10,7 @@ import { FormulaField } from './FormulaField';
 import { GetPagedAppsResponse } from './GetPagedAppsResponse';
 import { GetPagedFieldsResponse } from './GetPagedFieldsResponse';
 import { ListField } from './ListField';
+import { ListItemResponse } from './ListItemResponse';
 import { ListValue } from './ListValue';
 import { ReferenceField } from './ReferenceField';
 
@@ -258,6 +259,18 @@ export class ApiResponse<T> {
     );
   }
 
+  public asListItemResponseType(): ApiResponse<ListItemResponse> {
+    const apiResponse = this as ApiResponse<any>;
+
+    const listItemResponse = new ListItemResponse(apiResponse.data.id);
+
+    return new ApiResponse<ListItemResponse>(
+      apiResponse.statusCode,
+      apiResponse.message,
+      listItemResponse
+    );
+  }
+
   /**
    * @method asFileCollectionType - Converts the field item to the appropriate field object based upon the field item's type.
    * @param fieldItem - The field item to convert.
diff --git a/src/models/ListItemRequest.ts b/src/models/ListItemRequest.ts
new file mode 100644
index 0000000..5af2f65
--- /dev/null
+++ b/src/models/ListItemRequest.ts
@@ -0,0 +1,52 @@
+/**
+ * @class ListItemRequest - Represents a list item request
+ */
+export class ListItemRequest {
+  /**
+   * @property {number} listId - The id for the list that the item belongs to.
+   */
+  public listId: number;
+
+  /**
+   * @property {string} id - The id for the list item.
+   */
+  public id: string;
+
+  /**
+   * @property {string} name - The name of the list item.
+   */
+  public name: string;
+
+  /**
+   * @property {number} numericValue - The numeric value of the list item.
+   */
+  public numericValue: number;
+
+  /**
+   * @property {string} color - The color of the list item.
+   */
+  public color: string;
+
+  /**
+   * @constructor - Creates a new instance of the ListItemRequest class.
+   * @param {number} listId - The id for the list that the item belongs to.
+   * @param {string} id - The id for the list item.
+   * @param {string} name - The name of the list item.
+   * @param {number} numericValue - The numeric value of the list item.
+   * @param {string} color - The color of the list item.
+   * @returns {ListItemRequest} - A new instance of the ListItemRequest class.
+   */
+  constructor(
+    listId: number,
+    id: string,
+    name: string,
+    numericValue: number,
+    color: string
+  ) {
+    this.listId = listId;
+    this.id = id;
+    this.name = name;
+    this.numericValue = numericValue;
+    this.color = color;
+  }
+}
diff --git a/src/models/ListItemResponse.ts b/src/models/ListItemResponse.ts
new file mode 100644
index 0000000..4fc8b8b
--- /dev/null
+++ b/src/models/ListItemResponse.ts
@@ -0,0 +1,10 @@
+import { CreatedWithIdResponse } from './CreatedWithIdResponse';
+
+/**
+ * @class ListItemResponse - Represents a respons when a list item is created or updated.
+ */
+export class ListItemResponse extends CreatedWithIdResponse {
+  constructor(id: number) {
+    super(id);
+  }
+}
diff --git a/src/models/OnspringClient.ts b/src/models/OnspringClient.ts
index 193d792..ff6280f 100644
--- a/src/models/OnspringClient.ts
+++ b/src/models/OnspringClient.ts
@@ -14,6 +14,8 @@ import { type SaveFileRequest } from './SaveFileRequest';
 import { type CreatedWithIdResponse } from './CreatedWithIdResponse';
 import { type FileInfo } from './FileInfo';
 import { type File } from './File';
+import { type ListItemResponse } from './ListItemResponse';
+import { type ListItemRequest } from './ListItemRequest';
 
 /**
  * @class OnspringClient - A client that can communicate with the Onspring API.
@@ -269,6 +271,22 @@ export class OnspringClient {
     return apiResponse;
   }
 
+  public async addOrUpdateListItem(
+    listItemRequest: ListItemRequest
+  ): Promise<ApiResponse<ListItemResponse>> {
+    const { listId, ...data } = listItemRequest;
+
+    const endpoint = EndpointFactory.getAddOrUpdateListItemEndpoint(listId);
+
+    const apiResponse = await this.put<any>(endpoint, data);
+
+    if (apiResponse.isSuccessful === false) {
+      return apiResponse;
+    }
+
+    return apiResponse.asListItemResponseType();
+  }
+
   /**
    * @method get - Makes a GET request to the specified endpoint.
    * @param {string} endpoint - The endpoint that will be used to make the request.
@@ -301,6 +319,29 @@ export class OnspringClient {
     return apiResponse;
   }
 
+  /**
+   * @method put - Makes a PUT request to the specified endpoint.
+   * @param {string} endpoint - The endpoint that will be used to make the request.
+   * @param {any} data - The data that will be sent with the request.
+   * @param {AxiosRequestConfig} config - The configuration that will be used to make the request.
+   * @returns {Promise<ApiResponse<T>>} - A promise that resolves to an ApiResponse of type T.
+   */
+  private async put<T>(
+    endpoint: string,
+    data: any,
+    config: AxiosRequestConfig = {}
+  ): Promise<ApiResponse<T>> {
+    const response = await this._client.put(endpoint, data, config);
+    const apiResponse = ApiResponseFactory.getApiResponse<T>(response);
+    return apiResponse;
+  }
+
+  /**
+   * @method delete - Makes a DELETE request to the specified endpoint.
+   * @param {string} endpoint - The endpoint that will be used to make the request.
+   * @param {AxiosRequestConfig} config - The configuration that will be used to make the request.
+   * @returns {Promise<ApiResponse<T>>} - A promise that resolves to an ApiResponse of type T.
+   */
   private async delete<T>(
     endpoint: string,
     config: AxiosRequestConfig = {}
diff --git a/tests/ApiResponse.spec.ts b/tests/ApiResponse.spec.ts
index 4f97f10..3f296d1 100644
--- a/tests/ApiResponse.spec.ts
+++ b/tests/ApiResponse.spec.ts
@@ -18,6 +18,7 @@ import { File } from '../src/models/File';
 import fs from 'fs';
 import { type AxiosResponse } from 'axios';
 import path from 'path';
+import { ListItemResponse } from '../src/models/ListItemResponse';
 
 describe('ApiResponse', function () {
   it('should be defined', function () {
@@ -916,4 +917,29 @@ describe('ApiResponse', function () {
       expect(fileResponse.data).to.have.property('contentLength', 0);
     });
   });
+
+  describe('asListItemResponseType', function () {
+    it('should be defined', function () {
+      expect(ApiResponse.prototype.asListItemResponseType).to.not.be.undefined;
+    });
+
+    it('should be a function', function () {
+      expect(ApiResponse.prototype.asListItemResponseType).to.be.a('function');
+    });
+    it('should have no parameters', function () {
+      expect(ApiResponse.prototype.asListItemResponseType.length).to.equal(0);
+    });
+    it('should return an ApiResponse<ListItemResponse>', function () {
+      const apiResponse = new ApiResponse(200, 'OK', {
+        id: '3fa85f64-5717-4562-b3fc-2c963f66afa6',
+      });
+      const listItemResponse = apiResponse.asListItemResponseType();
+      expect(listItemResponse).to.be.an.instanceof(ApiResponse);
+      expect(listItemResponse.data).to.be.an.instanceof(ListItemResponse);
+      expect(listItemResponse.data).to.have.property(
+        'id',
+        '3fa85f64-5717-4562-b3fc-2c963f66afa6'
+      );
+    });
+  });
 });
diff --git a/tests/ListItemRequest.spec.ts b/tests/ListItemRequest.spec.ts
new file mode 100644
index 0000000..1ff2d67
--- /dev/null
+++ b/tests/ListItemRequest.spec.ts
@@ -0,0 +1,33 @@
+import { ListItemRequest } from '../src/models/ListItemRequest';
+import { expect } from 'chai';
+
+describe('ListItemRequest', function () {
+  it('should be defined', function () {
+    expect(ListItemRequest).to.not.be.undefined;
+  });
+
+  it('should have a constructor with 5 parameters', function () {
+    expect(ListItemRequest).to.have.property('constructor');
+    expect(ListItemRequest).to.have.lengthOf(5);
+  });
+
+  it('should have a constructor that takes a listId, id, name, numericValue, and color and sets the listId, id, name, numericValue, and color properties', function () {
+    const listId = 1;
+    const id = '1';
+    const name = 'name';
+    const numericValue = 1;
+    const color = 'color';
+    const listItemRequest = new ListItemRequest(
+      listId,
+      id,
+      name,
+      numericValue,
+      color
+    );
+    expect(listItemRequest).to.have.property('listId', listId);
+    expect(listItemRequest).to.have.property('id', id);
+    expect(listItemRequest).to.have.property('name', name);
+    expect(listItemRequest).to.have.property('numericValue', numericValue);
+    expect(listItemRequest).to.have.property('color', color);
+  });
+});
diff --git a/tests/ListItemResponse.spec.ts b/tests/ListItemResponse.spec.ts
new file mode 100644
index 0000000..9de321a
--- /dev/null
+++ b/tests/ListItemResponse.spec.ts
@@ -0,0 +1,19 @@
+import { ListItemResponse } from '../src/models/ListItemResponse';
+import { expect } from 'chai';
+
+describe('ListItemResponse', function () {
+  it('should be defined', function () {
+    expect(ListItemResponse).to.not.be.undefined;
+  });
+
+  it('should have a constructor with 1 parameter', function () {
+    expect(ListItemResponse).to.have.property('constructor');
+    expect(ListItemResponse.constructor).to.have.lengthOf(1);
+  });
+
+  it('should have a constructor that takes an id and sets the id property', function () {
+    const id = 1;
+    const listItemResponse = new ListItemResponse(id);
+    expect(listItemResponse).to.have.property('id', id);
+  });
+});
diff --git a/tests/OnspringClient.spec.ts b/tests/OnspringClient.spec.ts
index 2486ee6..071e809 100644
--- a/tests/OnspringClient.spec.ts
+++ b/tests/OnspringClient.spec.ts
@@ -20,6 +20,8 @@ import { FileInfo } from '../src/models/FileInfo';
 import { File } from '../src/models/File';
 import fs from 'fs';
 import path from 'path';
+import { ListItemRequest } from '../src/models/ListItemRequest';
+import { ListItemResponse } from '../src/models/ListItemResponse';
 
 describe('OnspringClient', function () {
   const baseUrl = 'https://api.onspring.dev';
@@ -2108,4 +2110,199 @@ describe('OnspringClient', function () {
       expect(result.data).to.be.null;
     });
   });
+
+  describe('addOrUpdateListItem', function () {
+    it('should be defined', function () {
+      expect(OnspringClient.prototype.addOrUpdateListItem).to.not.be.undefined;
+    });
+
+    it('should be a function', function () {
+      expect(OnspringClient.prototype.addOrUpdateListItem).to.be.a('function');
+    });
+
+    it('should return a promise', function () {
+      expect(
+        new OnspringClient(baseUrl, apiKey).addOrUpdateListItem(
+          new ListItemRequest(1, 'id', 'value', 1, 'color')
+        )
+      ).to.be.instanceOf(Promise);
+    });
+
+    it('should return a promise that resolves to an api response when request is successful at updating a list item', async function () {
+      const client = new OnspringClient(baseUrl, apiKey);
+
+      const mockAxiosClient = axios.create({
+        baseURL: baseUrl,
+        headers: {
+          'x-apikey': apiKey,
+          'x-api-version': '2',
+        },
+      });
+
+      sinon.stub(mockAxiosClient, 'put').returns(
+        Promise.resolve({
+          status: 200,
+          statusText: 'OK',
+          data: { id: '3fa85f64-5717-4562-b3fc-2c963f66afa6' },
+          headers: {},
+          config: {} as InternalAxiosRequestConfig,
+        } as AxiosResponse)
+      );
+
+      sinon.stub(client, '_client' as any).value(mockAxiosClient);
+
+      const result = await client.addOrUpdateListItem(
+        new ListItemRequest(1, 'id', 'value', 1, 'color')
+      );
+
+      expect(result).to.be.instanceOf(ApiResponse);
+      expect(result).to.have.property('statusCode', 200);
+      expect(result).to.have.property('isSuccessful', true);
+      expect(result).to.have.property('message', '');
+      expect(result.data).to.be.instanceOf(ListItemResponse);
+      expect(result.data).to.have.property(
+        'id',
+        '3fa85f64-5717-4562-b3fc-2c963f66afa6'
+      );
+    });
+
+    it('should return a promise that resolves to an api response when request is successful at adding a list item', async function () {
+      const client = new OnspringClient(baseUrl, apiKey);
+
+      const mockAxiosClient = axios.create({
+        baseURL: baseUrl,
+        headers: {
+          'x-apikey': apiKey,
+          'x-api-version': '2',
+        },
+      });
+
+      sinon.stub(mockAxiosClient, 'put').returns(
+        Promise.resolve({
+          status: 201,
+          statusText: 'OK',
+          data: { id: '3fa85f64-5717-4562-b3fc-2c963f66afa6' },
+          headers: {},
+          config: {} as InternalAxiosRequestConfig,
+        } as AxiosResponse)
+      );
+
+      sinon.stub(client, '_client' as any).value(mockAxiosClient);
+
+      const result = await client.addOrUpdateListItem(
+        new ListItemRequest(1, 'id', 'value', 1, 'color')
+      );
+
+      expect(result).to.be.instanceOf(ApiResponse);
+      expect(result).to.have.property('statusCode', 201);
+      expect(result).to.have.property('isSuccessful', true);
+      expect(result).to.have.property('message', '');
+      expect(result.data).to.be.instanceOf(ListItemResponse);
+      expect(result.data).to.have.property(
+        'id',
+        '3fa85f64-5717-4562-b3fc-2c963f66afa6'
+      );
+    });
+  });
+
+  it('should return a promise that resolves to an api response when request receives a 401 response', async function () {
+    const client = new OnspringClient(baseUrl, apiKey);
+
+    const mockAxiosClient = axios.create({
+      baseURL: baseUrl,
+      headers: {
+        'x-apikey': apiKey,
+        'x-api-version': '2',
+      },
+    });
+
+    sinon.stub(mockAxiosClient, 'put').returns(
+      Promise.resolve({
+        status: 401,
+        statusText: 'Unauthorized',
+        headers: {},
+        config: {} as InternalAxiosRequestConfig,
+      } as AxiosResponse)
+    );
+
+    sinon.stub(client, '_client' as any).value(mockAxiosClient);
+
+    const result = await client.addOrUpdateListItem(
+      new ListItemRequest(1, 'id', 'value', 1, 'color')
+    );
+
+    expect(result).to.be.instanceOf(ApiResponse);
+    expect(result).to.have.property('statusCode', 401);
+    expect(result).to.have.property('isSuccessful', false);
+    expect(result.message).to.be.undefined;
+    expect(result.data).to.be.null;
+  });
+
+  it('should return a promise that resolves to an api response when request receives a 403 response', async function () {
+    const client = new OnspringClient(baseUrl, apiKey);
+
+    const mockAxiosClient = axios.create({
+      baseURL: baseUrl,
+      headers: {
+        'x-apikey': apiKey,
+        'x-api-version': '2',
+      },
+    });
+
+    sinon.stub(mockAxiosClient, 'put').returns(
+      Promise.resolve({
+        status: 403,
+        statusText: 'Forbidden',
+        data: { message: 'Forbidden' },
+        headers: {},
+        config: {} as InternalAxiosRequestConfig,
+      } as AxiosResponse)
+    );
+
+    sinon.stub(client, '_client' as any).value(mockAxiosClient);
+
+    const result = await client.addOrUpdateListItem(
+      new ListItemRequest(1, 'id', 'value', 1, 'color')
+    );
+
+    expect(result).to.be.instanceOf(ApiResponse);
+    expect(result).to.have.property('statusCode', 403);
+    expect(result).to.have.property('isSuccessful', false);
+    expect(result).to.have.property('message', 'Forbidden');
+    expect(result.data).to.be.null;
+  });
+
+  it('should return a promise that resolves to an api response when request receives a 404 response', async function () {
+    const client = new OnspringClient(baseUrl, apiKey);
+
+    const mockAxiosClient = axios.create({
+      baseURL: baseUrl,
+      headers: {
+        'x-apikey': apiKey,
+        'x-api-version': '2',
+      },
+    });
+
+    sinon.stub(mockAxiosClient, 'put').returns(
+      Promise.resolve({
+        status: 404,
+        statusText: 'Not Found',
+        data: { message: 'Not Found' },
+        headers: {},
+        config: {} as InternalAxiosRequestConfig,
+      } as AxiosResponse)
+    );
+
+    sinon.stub(client, '_client' as any).value(mockAxiosClient);
+
+    const result = await client.addOrUpdateListItem(
+      new ListItemRequest(1, 'id', 'value', 1, 'color')
+    );
+
+    expect(result).to.be.instanceOf(ApiResponse);
+    expect(result).to.have.property('statusCode', 404);
+    expect(result).to.have.property('isSuccessful', false);
+    expect(result).to.have.property('message', 'Not Found');
+    expect(result.data).to.be.null;
+  });
 });