diff --git a/README.md b/README.md
index e1b9a2f..a8ee706 100644
--- a/README.md
+++ b/README.md
@@ -196,7 +196,7 @@ RdtClient also emulates part of the SABnzbd API so Sonarr and Radarr can add NZB
 1. Enter `6500` in the `Port` field.
 1. Enable `Use SSL` only if you access RdtClient through HTTPS.
 1. Leave `URL Base` empty unless RdtClient is configured with a `BasePath`, for example `/rdt`.
-1. If RdtClient authentication is enabled, leave `API Key` empty and enter your RdtClient username and password.
+1. If RdtClient authentication is enabled, leave `API Key` empty and enter your RdtClient username and password. If your client only supports an API key, enter `{username}:{password}` in `API Key`.
 1. If RdtClient authentication is disabled, enter any value in `API Key`, for example `rdtclient`, and leave username/password empty.
 1. Set the category to `sonarr` for Sonarr or `radarr` for Radarr.
 1. Hit `Test` and then `Save` if all is well.
diff --git a/server/RdtClient.Service/Middleware/SabnzbdHandler.cs b/server/RdtClient.Service/Middleware/SabnzbdHandler.cs
index 2779faf..6fab68f 100644
--- a/server/RdtClient.Service/Middleware/SabnzbdHandler.cs
+++ b/server/RdtClient.Service/Middleware/SabnzbdHandler.cs
@@ -65,6 +65,36 @@ public class SabnzbdHandler(Authentication authentication, IHttpContextAccessor
                 return;
             }
 
+            var apiKey = GetParam("apikey");
+
+            if (!String.IsNullOrWhiteSpace(apiKey))
+            {
+                var separatorIndex = apiKey.IndexOf(':');
+
+                if (separatorIndex <= 0 || separatorIndex == apiKey.Length - 1)
+                {
+                    context.Fail();
+
+                    return;
+                }
+
+                var username = apiKey[..separatorIndex];
+                var password = apiKey[(separatorIndex + 1)..];
+
+                var loginResult = await authentication.Login(username, password);
+
+                if (loginResult.Succeeded)
+                {
+                    context.Succeed(requirement);
+
+                    return;
+                }
+
+                context.Fail();
+
+                return;
+            }
+
             // Authentication required but missing credentials
             context.Fail();
         }
diff --git a/server/RdtClient.Web.Test/Controllers/SabnzbdHandlerTest.cs b/server/RdtClient.Web.Test/Controllers/SabnzbdHandlerTest.cs
index 313427b..efcd52c 100644
--- a/server/RdtClient.Web.Test/Controllers/SabnzbdHandlerTest.cs
+++ b/server/RdtClient.Web.Test/Controllers/SabnzbdHandlerTest.cs
@@ -64,6 +64,110 @@ public class SabnzbdHandlerTest
         Assert.True(context.HasSucceeded, "HasSucceeded should be true for valid credentials");
     }
 
+    [Fact]
+    public async Task HandleAsync_ValidApiKeyCredentials_Succeeds()
+    {
+        // Arrange
+        _settings.Current.General.AuthenticationType = AuthenticationType.UserNamePassword;
+        var httpContext = new DefaultHttpContext
+        {
+            Request =
+            {
+                QueryString = new("?apikey=user:pass")
+            }
+        };
+
+        _httpContextAccessorMock.Setup(a => a.HttpContext).Returns(httpContext);
+
+        var context = CreateContext(httpContext);
+        _authenticationMock.Setup(a => a.Login("user", "pass")).ReturnsAsync(SignInResult.Success);
+
+        // Act
+        await _handler.HandleAsync(context);
+
+        // Assert
+        Assert.True(context.HasSucceeded, "HasSucceeded should be true for valid api key credentials");
+    }
+
+    [Fact]
+    public async Task HandleAsync_ValidApiKeyCredentialsFromForm_Succeeds()
+    {
+        // Arrange
+        _settings.Current.General.AuthenticationType = AuthenticationType.UserNamePassword;
+        var httpContext = new DefaultHttpContext();
+        httpContext.Request.ContentType = "application/x-www-form-urlencoded";
+        httpContext.Request.Form = new FormCollection(new()
+        {
+            {
+                "apikey", "user:pass"
+            }
+        });
+
+        _httpContextAccessorMock.Setup(a => a.HttpContext).Returns(httpContext);
+
+        var context = CreateContext(httpContext);
+        _authenticationMock.Setup(a => a.Login("user", "pass")).ReturnsAsync(SignInResult.Success);
+
+        // Act
+        await _handler.HandleAsync(context);
+
+        // Assert
+        Assert.True(context.HasSucceeded, "HasSucceeded should be true for valid form api key credentials");
+    }
+
+    [Fact]
+    public async Task HandleAsync_ApiKeyPasswordContainingColon_Succeeds()
+    {
+        // Arrange
+        _settings.Current.General.AuthenticationType = AuthenticationType.UserNamePassword;
+        var httpContext = new DefaultHttpContext
+        {
+            Request =
+            {
+                QueryString = new("?apikey=user:pass:with:colons")
+            }
+        };
+
+        _httpContextAccessorMock.Setup(a => a.HttpContext).Returns(httpContext);
+
+        var context = CreateContext(httpContext);
+        _authenticationMock.Setup(a => a.Login("user", "pass:with:colons")).ReturnsAsync(SignInResult.Success);
+
+        // Act
+        await _handler.HandleAsync(context);
+
+        // Assert
+        Assert.True(context.HasSucceeded, "HasSucceeded should be true when the password contains colons");
+    }
+
+    [Fact]
+    public async Task HandleAsync_MaCredentialsTakePrecedenceOverApiKey()
+    {
+        // Arrange
+        _settings.Current.General.AuthenticationType = AuthenticationType.UserNamePassword;
+        var httpContext = new DefaultHttpContext
+        {
+            Request =
+            {
+                QueryString = new("?ma_username=user&ma_password=wrong&apikey=user:pass")
+            }
+        };
+
+        _httpContextAccessorMock.Setup(a => a.HttpContext).Returns(httpContext);
+
+        var context = CreateContext(httpContext);
+        _authenticationMock.Setup(a => a.Login("user", "wrong")).ReturnsAsync(SignInResult.Failed);
+        _authenticationMock.Setup(a => a.Login("user", "pass")).ReturnsAsync(SignInResult.Success);
+
+        // Act
+        await _handler.HandleAsync(context);
+
+        // Assert
+        Assert.False(context.HasSucceeded, "HasSucceeded should be false because ma_username/ma_password take precedence");
+        _authenticationMock.Verify(a => a.Login("user", "wrong"), Times.Once);
+        _authenticationMock.Verify(a => a.Login("user", "pass"), Times.Never);
+    }
+
     [Fact]
     public async Task HandleAsync_AlreadyAuthenticated_Succeeds()
     {
@@ -108,6 +212,59 @@ public class SabnzbdHandlerTest
         Assert.False(context.HasSucceeded, "HasSucceeded should be false for invalid credentials");
     }
 
+    [Fact]
+    public async Task HandleAsync_InvalidApiKeyCredentials_DoesNotSucceed()
+    {
+        // Arrange
+        _settings.Current.General.AuthenticationType = AuthenticationType.UserNamePassword;
+        var httpContext = new DefaultHttpContext
+        {
+            Request =
+            {
+                QueryString = new("?apikey=user:wrong")
+            }
+        };
+
+        _httpContextAccessorMock.Setup(a => a.HttpContext).Returns(httpContext);
+
+        var context = CreateContext(httpContext);
+        _authenticationMock.Setup(a => a.Login("user", "wrong")).ReturnsAsync(SignInResult.Failed);
+
+        // Act
+        await _handler.HandleAsync(context);
+
+        // Assert
+        Assert.False(context.HasSucceeded, "HasSucceeded should be false for invalid api key credentials");
+    }
+
+    [Theory]
+    [InlineData("missingdelimiter")]
+    [InlineData(":pass")]
+    [InlineData("user:")]
+    public async Task HandleAsync_MalformedApiKey_DoesNotSucceed(String apiKey)
+    {
+        // Arrange
+        _settings.Current.General.AuthenticationType = AuthenticationType.UserNamePassword;
+        var httpContext = new DefaultHttpContext
+        {
+            Request =
+            {
+                QueryString = new($"?apikey={apiKey}")
+            }
+        };
+
+        _httpContextAccessorMock.Setup(a => a.HttpContext).Returns(httpContext);
+
+        var context = CreateContext(httpContext);
+
+        // Act
+        await _handler.HandleAsync(context);
+
+        // Assert
+        Assert.False(context.HasSucceeded, "HasSucceeded should be false for malformed api keys");
+        _authenticationMock.Verify(a => a.Login(It.IsAny<String>(), It.IsAny<String>()), Times.Never);
+    }
+
     [Fact]
     public async Task HandleAsync_MissingCredentials_DoesNotSucceed()
     {