diff --git a/SafeExamBrowser.UserInterface.Desktop/BrowserWindow.xaml.cs b/SafeExamBrowser.UserInterface.Desktop/BrowserWindow.xaml.cs
index fd0edf05..789ade1b 100644
--- a/SafeExamBrowser.UserInterface.Desktop/BrowserWindow.xaml.cs
+++ b/SafeExamBrowser.UserInterface.Desktop/BrowserWindow.xaml.cs
@@ -142,6 +142,14 @@ namespace SafeExamBrowser.UserInterface.Desktop
 			}
 		}
 
+		private void BrowserWindow_Loaded(object sender, RoutedEventArgs e)
+		{
+			if (isMainWindow)
+			{
+				WindowUtility.DisableCloseButtonFor(this);
+			}
+		}
+
 		private CustomPopupPlacement[] MenuPopup_PlacementCallback(Size popupSize, Size targetSize, Point offset)
 		{
 			return new[]
@@ -196,6 +204,7 @@ namespace SafeExamBrowser.UserInterface.Desktop
 			BackwardButton.Click += (o, args) => BackwardNavigationRequested?.Invoke();
 			Closing += BrowserWindow_Closing;
 			ForwardButton.Click += (o, args) => ForwardNavigationRequested?.Invoke();
+			Loaded += BrowserWindow_Loaded;
 			MenuButton.Click += (o, args) => MenuPopup.IsOpen = !MenuPopup.IsOpen;
 			MenuButton.MouseLeave += (o, args) => Task.Delay(250).ContinueWith(_ => Dispatcher.Invoke(() => MenuPopup.IsOpen = MenuPopup.IsMouseOver));
 			MenuPopup.CustomPopupPlacementCallback = new CustomPopupPlacementCallback(MenuPopup_PlacementCallback);
diff --git a/SafeExamBrowser.UserInterface.Mobile/BrowserWindow.xaml.cs b/SafeExamBrowser.UserInterface.Mobile/BrowserWindow.xaml.cs
index f05ef415..019ae83f 100644
--- a/SafeExamBrowser.UserInterface.Mobile/BrowserWindow.xaml.cs
+++ b/SafeExamBrowser.UserInterface.Mobile/BrowserWindow.xaml.cs
@@ -142,6 +142,14 @@ namespace SafeExamBrowser.UserInterface.Mobile
 			}
 		}
 
+		private void BrowserWindow_Loaded(object sender, RoutedEventArgs e)
+		{
+			if (isMainWindow)
+			{
+				WindowUtility.DisableCloseButtonFor(this);
+			}
+		}
+
 		private CustomPopupPlacement[] MenuPopup_PlacementCallback(Size popupSize, Size targetSize, Point offset)
 		{
 			return new[]
@@ -196,6 +204,7 @@ namespace SafeExamBrowser.UserInterface.Mobile
 			BackwardButton.Click += (o, args) => BackwardNavigationRequested?.Invoke();
 			Closing += BrowserWindow_Closing;
 			ForwardButton.Click += (o, args) => ForwardNavigationRequested?.Invoke();
+			Loaded += BrowserWindow_Loaded;
 			MenuButton.Click += (o, args) => MenuPopup.IsOpen = !MenuPopup.IsOpen;
 			MenuButton.MouseLeave += (o, args) => Task.Delay(250).ContinueWith(_ => Dispatcher.Invoke(() => MenuPopup.IsOpen = MenuPopup.IsMouseOver));
 			MenuPopup.CustomPopupPlacementCallback = new CustomPopupPlacementCallback(MenuPopup_PlacementCallback);
diff --git a/SafeExamBrowser.UserInterface.Shared/SafeExamBrowser.UserInterface.Shared.csproj b/SafeExamBrowser.UserInterface.Shared/SafeExamBrowser.UserInterface.Shared.csproj
index 3b9fe27f..54a92da2 100644
--- a/SafeExamBrowser.UserInterface.Shared/SafeExamBrowser.UserInterface.Shared.csproj
+++ b/SafeExamBrowser.UserInterface.Shared/SafeExamBrowser.UserInterface.Shared.csproj
@@ -67,6 +67,7 @@
     <Compile Include="Utilities\IconResourceLoader.cs" />
     <Compile Include="Utilities\UrlRandomizer.cs" />
     <Compile Include="Utilities\VisualExtensions.cs" />
+    <Compile Include="Utilities\WindowUtility.cs" />
     <Compile Include="Utilities\XamlIconResource.cs" />
   </ItemGroup>
   <ItemGroup>
diff --git a/SafeExamBrowser.UserInterface.Shared/Utilities/WindowUtility.cs b/SafeExamBrowser.UserInterface.Shared/Utilities/WindowUtility.cs
new file mode 100644
index 00000000..69fb622c
--- /dev/null
+++ b/SafeExamBrowser.UserInterface.Shared/Utilities/WindowUtility.cs
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ * 
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+using System;
+using System.Runtime.InteropServices;
+using System.Windows;
+using System.Windows.Interop;
+
+namespace SafeExamBrowser.UserInterface.Shared.Utilities
+{
+	public static class WindowUtility
+	{
+		private const uint MF_BYCOMMAND = 0x00000000;
+		private const uint MF_GRAYED = 0x00000001;
+		private const uint MF_ENABLED = 0x00000000;
+		private const uint SC_CLOSE = 0xF060;
+
+		public static void DisableCloseButtonFor(Window window)
+		{
+			var handle = new WindowInteropHelper(window);
+			var systemMenu = GetSystemMenu(handle.Handle, false);
+
+			if (systemMenu != IntPtr.Zero)
+			{
+				EnableMenuItem(systemMenu, SC_CLOSE, MF_BYCOMMAND | MF_GRAYED);
+			}
+		}
+
+		[DllImport("user32.dll")]
+		private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);
+
+		[DllImport("user32.dll")]
+		private static extern bool EnableMenuItem(IntPtr hMenu, uint uIDEnableItem, uint uEnable);
+	}
+}