diff --git a/dotnet/src/webdriver/BiDi/BrowsingContext/SetViewportCommand.cs b/dotnet/src/webdriver/BiDi/BrowsingContext/SetViewportCommand.cs
index 90c493a0de6d5..9021c64551106 100644
--- a/dotnet/src/webdriver/BiDi/BrowsingContext/SetViewportCommand.cs
+++ b/dotnet/src/webdriver/BiDi/BrowsingContext/SetViewportCommand.cs
@@ -17,16 +17,19 @@
 // under the License.
 // </copyright>
 
+using OpenQA.Selenium.BiDi.Json.Converters;
+using System.Text.Json.Serialization;
+
 namespace OpenQA.Selenium.BiDi.BrowsingContext;
 
 internal sealed class SetViewportCommand(SetViewportParameters @params)
     : Command<SetViewportParameters, SetViewportResult>(@params, "browsingContext.setViewport");
 
-internal sealed record SetViewportParameters(BrowsingContext Context, Viewport? Viewport, double? DevicePixelRatio) : Parameters;
+internal sealed record SetViewportParameters(BrowsingContext Context, [property: JsonConverter(typeof(OptionalConverter<Viewport?>))] Optional<Viewport?>? Viewport, double? DevicePixelRatio) : Parameters;
 
 public sealed class SetViewportOptions : CommandOptions
 {
-    public Viewport? Viewport { get; set; }
+    public Optional<Viewport?>? Viewport { get; set; }
 
     public double? DevicePixelRatio { get; set; }
 }
diff --git a/dotnet/src/webdriver/BiDi/Json/Converters/OptionalConverter.cs b/dotnet/src/webdriver/BiDi/Json/Converters/OptionalConverter.cs
new file mode 100644
index 0000000000000..0fa368adce1f9
--- /dev/null
+++ b/dotnet/src/webdriver/BiDi/Json/Converters/OptionalConverter.cs
@@ -0,0 +1,51 @@
+// <copyright file="OptionalConverter.cs" company="Selenium Committers">
+// Licensed to the Software Freedom Conservancy (SFC) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The SFC licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+// </copyright>
+
+using System;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace OpenQA.Selenium.BiDi.Json.Converters;
+
+public sealed class OptionalConverter<T> : JsonConverter<Optional<T>>
+{
+    public override Optional<T> Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType == JsonTokenType.Null)
+        {
+            reader.Read(); // consume null
+            return new Optional<T>(default!);
+        }
+
+        T value = JsonSerializer.Deserialize<T>(ref reader, options)!;
+        return new Optional<T>(value);
+    }
+
+    public override void Write(Utf8JsonWriter writer, Optional<T> value, JsonSerializerOptions options)
+    {
+        if (value.TryGetValue(out var optionalValue))
+        {
+            JsonSerializer.Serialize(writer, optionalValue, options);
+        }
+        else
+        {
+            writer.WriteNullValue();
+        }
+    }
+}
diff --git a/dotnet/src/webdriver/BiDi/Optional.cs b/dotnet/src/webdriver/BiDi/Optional.cs
new file mode 100644
index 0000000000000..e31fbbce2c9c6
--- /dev/null
+++ b/dotnet/src/webdriver/BiDi/Optional.cs
@@ -0,0 +1,47 @@
+// <copyright file="Optional.cs" company="Selenium Committers">
+// Licensed to the Software Freedom Conservancy (SFC) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The SFC licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+// </copyright>
+
+using System;
+
+namespace OpenQA.Selenium.BiDi;
+
+public readonly record struct Optional<T>
+{
+    private readonly T _value;
+    public bool HasValue { get; }
+
+    public T Value => HasValue
+        ? _value
+        : throw new InvalidOperationException("Optional has no value. Check IsSet first.");
+
+    public Optional(T value)
+    {
+        _value = value;
+        HasValue = true;
+    }
+
+    public bool TryGetValue(out T value)
+    {
+        value = _value;
+        return HasValue;
+    }
+
+    // implicit conversion from T -> Optional<T>
+    public static implicit operator Optional<T>(T value) => new(value);
+}
diff --git a/dotnet/test/common/BiDi/BrowsingContext/BrowsingContextTest.cs b/dotnet/test/common/BiDi/BrowsingContext/BrowsingContextTest.cs
index a1a8cda5924b8..9a62004bede8b 100644
--- a/dotnet/test/common/BiDi/BrowsingContext/BrowsingContextTest.cs
+++ b/dotnet/test/common/BiDi/BrowsingContext/BrowsingContextTest.cs
@@ -298,13 +298,40 @@ public async Task CanCaptureScreenshotOfElement()
     [Test]
     public async Task CanSetViewport()
     {
-        await context.SetViewportAsync(new() { Viewport = new(250, 300) });
+        Task<int> GetWidthAsync() => context.Script.EvaluateAsync<int>("window.innerWidth", false);
+        Task<int> GetHeightAsync() => context.Script.EvaluateAsync<int>("window.innerHeight", false);
+
+        var defaultWidth = await GetWidthAsync();
+        var defaultHeight = await GetHeightAsync();
+
+        await context.SetViewportAsync(new() { Viewport = new Viewport(250, 300) });
+
+        Assert.That(await GetWidthAsync(), Is.EqualTo(250));
+        Assert.That(await GetHeightAsync(), Is.EqualTo(300));
+
+        await context.SetViewportAsync(new() { Viewport = new Viewport(250, 300) });
+        await context.SetViewportAsync(); // Sends nothing
+
+        Assert.That(await GetWidthAsync(), Is.EqualTo(250));
+        Assert.That(await GetHeightAsync(), Is.EqualTo(300));
+
+        await context.SetViewportAsync(new() { Viewport = new Viewport(250, 300) });
+        await context.SetViewportAsync(new() { Viewport = default }); // Sends nothing
+
+        Assert.That(await GetWidthAsync(), Is.EqualTo(250));
+        Assert.That(await GetHeightAsync(), Is.EqualTo(300));
+
+        await context.SetViewportAsync(new() { Viewport = new Viewport(250, 300) });
+        await context.SetViewportAsync(new() { Viewport = default(Viewport?) }); // Explicitly sends "null", resetting to default
+
+        Assert.That(await GetWidthAsync(), Is.EqualTo(defaultWidth));
+        Assert.That(await GetHeightAsync(), Is.EqualTo(defaultHeight));
     }
 
     [Test]
     public async Task CanSetViewportWithDevicePixelRatio()
     {
-        await context.SetViewportAsync(new() { Viewport = new(250, 300), DevicePixelRatio = 5 });
+        await context.SetViewportAsync(new() { Viewport = new Viewport(250, 300), DevicePixelRatio = 5 });
     }
 
     [Test]