diff --git a/ILSpy.Tests.Windows/WindowsSystemFontTests.cs b/ILSpy.Tests.Windows/WindowsSystemFontTests.cs
new file mode 100644
index 0000000000..39641a4322
--- /dev/null
+++ b/ILSpy.Tests.Windows/WindowsSystemFontTests.cs
@@ -0,0 +1,44 @@
+// Copyright (c) 2026 Christoph Wille
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using ICSharpCode.ILSpy.Util;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests.Windows;
+
+/// <summary>
+/// Verifies the NONCLIENTMETRICS message-font reader used to follow the Windows system UI font.
+/// The exact values depend on the machine (the accessibility "Text size" setting scales them),
+/// so the assertions only pin down what must hold everywhere: a face name is present and the
+/// size is a sane DIP value (metrics are requested at 96 DPI, so 12 on a default install,
+/// larger when text scaling is active).
+/// </summary>
+[TestFixture]
+public class WindowsSystemFontTests
+{
+	[Test]
+	public void MessageFontIsReadable()
+	{
+		bool available = WindowsSystemFont.TryGetMessageFont(out var faceName, out var fontSize);
+
+		Assert.That(available, Is.True);
+		Assert.That(faceName, Is.Not.Null.And.Not.Empty);
+		Assert.That(fontSize, Is.InRange(9.0, 48.0));
+	}
+}
diff --git a/ILSpy.Tests/UiFontTests.cs b/ILSpy.Tests/UiFontTests.cs
new file mode 100644
index 0000000000..a8a6b6768d
--- /dev/null
+++ b/ILSpy.Tests/UiFontTests.cs
@@ -0,0 +1,83 @@
+// Copyright (c) 2026 Christoph Wille
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless.NUnit;
+using Avalonia.Styling;
+
+using AwesomeAssertions;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests;
+
+/// <summary>
+/// Exercises App.ApplyUiFont (the system-UI-font override) with a synthetic size, since the
+/// production path reads machine-dependent NONCLIENTMETRICS. The interesting case is
+/// ContextMenu: the Simple theme pins its FontSize to the theme's FontSizeNormal resource,
+/// which outranks the inherited value that covers windows and menu-bar dropdowns, so a
+/// context menu silently stays at 12 unless the override also reaches that resource.
+/// </summary>
+[TestFixture]
+public class UiFontTests
+{
+	// Distinct from every size the Simple theme uses (10/12/16) so a match proves the override.
+	const double TestFontSize = 20;
+
+	[AvaloniaTest]
+	public void UiFontReachesWindowAndContextMenu()
+	{
+		var app = Application.Current!;
+		int styleCount = app.Styles.Count;
+		bool hadFontSizeNormal = app.Resources.TryGetValue("FontSizeNormal", out var previousFontSizeNormal);
+
+		try
+		{
+			App.ApplyUiFont(app, "Segoe UI", TestFontSize);
+
+			var contextMenu = new ContextMenu {
+				Items = { new MenuItem { Header = "Item" } },
+			};
+			var target = new Button { ContextMenu = contextMenu };
+			var window = new Window { Content = target };
+			window.Show();
+
+			window.FontSize.Should().Be(TestFontSize, "the TopLevel style must reach windows");
+
+			contextMenu.Open(target);
+			contextMenu.FontSize.Should().Be(TestFontSize,
+				"the override must beat the Simple theme's FontSizeNormal pin on ContextMenu");
+
+			window.Close();
+		}
+		finally
+		{
+			// App styles/resources are per-assembly shared state (ResetAppState rebuilds the MEF
+			// container, not the Application); undo so later tests keep the default 12.
+			while (app.Styles.Count > styleCount)
+				app.Styles.RemoveAt(app.Styles.Count - 1);
+			if (hadFontSizeNormal)
+				app.Resources["FontSizeNormal"] = previousFontSizeNormal;
+			else
+				app.Resources.Remove("FontSizeNormal");
+		}
+	}
+}
diff --git a/ILSpy/App.axaml.cs b/ILSpy/App.axaml.cs
index ba66e8aacd..57dcdac1c4 100644
--- a/ILSpy/App.axaml.cs
+++ b/ILSpy/App.axaml.cs
@@ -23,8 +23,12 @@
 using System.Threading.Tasks;
 
 using Avalonia;
+using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Controls.Primitives;
 using Avalonia.Markup.Xaml;
+using Avalonia.Media;
+using Avalonia.Styling;
 using Avalonia.Threading;
 
 using ICSharpCode.ILSpyX.Settings;
@@ -32,6 +36,7 @@
 using ICSharpCode.ILSpy.AppEnv;
 using ICSharpCode.ILSpy.AssemblyTree;
 using ICSharpCode.ILSpy.Themes;
+using ICSharpCode.ILSpy.Util;
 using ICSharpCode.ILSpy.Views;
 
 namespace ICSharpCode.ILSpy
@@ -85,6 +90,8 @@ public override void OnFrameworkInitializationCompleted()
 				StartupExceptions.Items.Add(new ExceptionData(ex));
 			}
 
+			ApplySystemFont();
+
 			if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
 			{
 				MainWindow? mainWindow = null;
@@ -123,6 +130,49 @@ public override void OnFrameworkInitializationCompleted()
 			base.OnFrameworkInitializationCompleted();
 		}
 
+		// Follows the Windows system UI font instead of Avalonia's built-in Segoe UI 12 default.
+		// The two coincide on a stock install, but the accessibility "Text size" setting scales
+		// the system metrics without changing DPI, and only apps that read them honor it.
+		// No-op on non-Windows platforms.
+		//
+		// The following deliberately do NOT follow the system font:
+		//  - the decompiled-code editor: user-configurable via DisplaySettings.SelectedFont/Size;
+		//  - controls with an explicit FontSize in their XAML: search-pane results, the resource
+		//    string/object tables (whose fixed RowHeight is tuned to that size), the zoom-buttons
+		//    overlay, the NuGet feed dialog, the XML-doc renderer, and the startup-error /
+		//    assertion dialogs;
+		//  - Simple-theme controls sized via FontSizeSmall/FontSizeLarge rather than
+		//    FontSizeNormal: TabItem headers (Options dialog, already larger than body text)
+		//    and Calendar buttons (unused in ILSpy).
+		void ApplySystemFont()
+		{
+			if (!WindowsSystemFont.TryGetMessageFont(out var faceName, out var fontSize))
+				return;
+
+			ApplyUiFont(this, faceName, fontSize);
+			AppLog.Mark($"System font applied: {faceName} @ {fontSize}");
+		}
+
+		// Styled on every TopLevel (not just Window) so popup surfaces -- menu dropdowns, context
+		// menus, tooltips -- get the font directly rather than relying on inheritance across the
+		// popup boundary. Internal so the headless test suite can exercise it with a synthetic
+		// size instead of the machine-dependent system metrics.
+		internal static void ApplyUiFont(Application app, string faceName, double fontSize)
+		{
+			var fontStyle = new Style(x => x.Is<TopLevel>());
+			fontStyle.Setters.Add(new Setter(TemplatedControl.FontFamilyProperty, new FontFamily(faceName)));
+			fontStyle.Setters.Add(new Setter(TemplatedControl.FontSizeProperty, fontSize));
+			app.Styles.Add(fontStyle);
+
+			// The Simple theme pins FontSize on a few control themes via its FontSizeNormal
+			// resource (ContextMenu is the one ILSpy hits; also PopupRoot/Window, where the
+			// style above already outranks it). A control-theme setter beats the inherited
+			// value, so shadow the resource at app level -- application resources win the
+			// DynamicResource lookup over theme resources. FontSizeSmall/FontSizeLarge stay
+			// untouched; their consumers are listed on ApplySystemFont.
+			app.Resources["FontSizeNormal"] = fontSize;
+		}
+
 		// Resolves where ILSpy.xml is loaded from. Shared by normal startup and the single-instance
 		// gate in Program.Main, which reads the settings before Avalonia starts.
 		internal static void ConfigureSettingsFilePathProvider(CommandLineArguments commandLineArguments)
diff --git a/ILSpy/Util/WindowsSystemFont.cs b/ILSpy/Util/WindowsSystemFont.cs
new file mode 100644
index 0000000000..07524bae43
--- /dev/null
+++ b/ILSpy/Util/WindowsSystemFont.cs
@@ -0,0 +1,116 @@
+// Copyright (c) 2026 Christoph Wille
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+using System.Runtime.InteropServices;
+
+#pragma warning disable CA1060 // Move pinvokes to native methods class
+
+namespace ICSharpCode.ILSpy.Util
+{
+	/// <summary>
+	/// Reads the Windows system UI font (the NONCLIENTMETRICS message font). Avalonia's built-in
+	/// default (Segoe UI at 12 DIP) matches a stock Windows install, but the accessibility
+	/// "Text size" setting (Settings &gt; Accessibility &gt; Text size) scales these metrics
+	/// without changing DPI, and only apps that read them follow it.
+	/// </summary>
+	public static partial class WindowsSystemFont
+	{
+		const uint SPI_GETNONCLIENTMETRICS = 0x0029;
+
+		[StructLayout(LayoutKind.Sequential)]
+		unsafe struct LOGFONTW
+		{
+			public int lfHeight;
+			public int lfWidth;
+			public int lfEscapement;
+			public int lfOrientation;
+			public int lfWeight;
+			public byte lfItalic;
+			public byte lfUnderline;
+			public byte lfStrikeOut;
+			public byte lfCharSet;
+			public byte lfOutPrecision;
+			public byte lfClipPrecision;
+			public byte lfQuality;
+			public byte lfPitchAndFamily;
+			public fixed char lfFaceName[32];
+		}
+
+		[StructLayout(LayoutKind.Sequential)]
+		struct NONCLIENTMETRICSW
+		{
+			public uint cbSize;
+			public int iBorderWidth;
+			public int iScrollWidth;
+			public int iScrollHeight;
+			public int iCaptionWidth;
+			public int iCaptionHeight;
+			public LOGFONTW lfCaptionFont;
+			public int iSmCaptionWidth;
+			public int iSmCaptionHeight;
+			public LOGFONTW lfSmCaptionFont;
+			public int iMenuWidth;
+			public int iMenuHeight;
+			public LOGFONTW lfMenuFont;
+			public LOGFONTW lfStatusFont;
+			public LOGFONTW lfMessageFont;
+			public int iPaddedBorderWidth;
+		}
+
+		[LibraryImport("user32.dll", SetLastError = true)]
+		[return: MarshalAs(UnmanagedType.Bool)]
+		private static partial bool SystemParametersInfoForDpi(uint uiAction, uint uiParam, ref NONCLIENTMETRICSW pvParam, uint fWinIni, uint dpi);
+
+		/// <summary>
+		/// Gets the system message font as an Avalonia-ready (family name, size in DIPs) pair.
+		/// Returns false on non-Windows platforms or if the metrics cannot be read. The metrics
+		/// are requested at 96 DPI, so the size is in device-independent pixels regardless of
+		/// display scaling (which Avalonia applies separately).
+		/// </summary>
+		public static unsafe bool TryGetMessageFont(out string faceName, out double fontSize)
+		{
+			faceName = string.Empty;
+			fontSize = 0;
+
+			// SystemParametersInfoForDpi requires Windows 10 1607.
+			if (!OperatingSystem.IsWindowsVersionAtLeast(10, 0, 14393))
+				return false;
+
+			var metrics = new NONCLIENTMETRICSW { cbSize = (uint)sizeof(NONCLIENTMETRICSW) };
+			if (!SystemParametersInfoForDpi(SPI_GETNONCLIENTMETRICS, metrics.cbSize, ref metrics, 0, 96))
+				return false;
+
+			// A negative lfHeight is the character height (the usual case for the message font);
+			// a positive one is the cell height. Either way the magnitude is the pixel size at
+			// the requested DPI.
+			int height = Math.Abs(metrics.lfMessageFont.lfHeight);
+			if (height <= 0)
+				return false;
+
+			var nameBuffer = new ReadOnlySpan<char>(metrics.lfMessageFont.lfFaceName, 32);
+			int terminator = nameBuffer.IndexOf('\0');
+			faceName = new string(terminator >= 0 ? nameBuffer[..terminator] : nameBuffer);
+			if (faceName.Length == 0)
+				return false;
+
+			fontSize = height;
+			return true;
+		}
+	}
+}