diff --git a/Docs/RELEASE_NOTES.md b/Docs/RELEASE_NOTES.md
index 3c7ba1c..942f700 100644
--- a/Docs/RELEASE_NOTES.md
+++ b/Docs/RELEASE_NOTES.md
@@ -2,7 +2,8 @@
 
 ## Unreleased
 
-No changes yet.
+- Adds a File menu entry that registers Margin as the default editor for
+  Markdown (.md) files through Launch Services.
 
 ## 0.5.1 — native reading and review fixes
 
diff --git a/Docs/screenshots/default-markdown-editor-menu.png b/Docs/screenshots/default-markdown-editor-menu.png
new file mode 100644
index 0000000..40f3f34
Binary files /dev/null and b/Docs/screenshots/default-markdown-editor-menu.png differ
diff --git a/Sources/MarginApp/AppDelegate.swift b/Sources/MarginApp/AppDelegate.swift
index 09ec11b..ddfbeca 100644
--- a/Sources/MarginApp/AppDelegate.swift
+++ b/Sources/MarginApp/AppDelegate.swift
@@ -1,5 +1,6 @@
 import AppKit
 import MarginCore
+import UniformTypeIdentifiers
 
 protocol WorkspacePathRenameParticipating: AnyObject {
     var documentURLForPathRename: URL? { get }
@@ -220,6 +221,25 @@ final class AppDelegate: NSObject, NSApplicationDelegate, NSMenuItemValidation {
         }
     }
 
+    @objc func setAsDefaultMarkdownEditor(_ sender: Any?) {
+        guard let markdownType = UTType(filenameExtension: "md") else { return }
+        let bundleURL = Bundle.main.bundleURL
+        let workspace = NSWorkspace.shared
+        workspace.setDefaultApplication(at: bundleURL, toOpen: markdownType) { error in
+            DispatchQueue.main.async {
+                let alert = NSAlert()
+                if error == nil {
+                    alert.messageText = "Margin is now the default editor for Markdown files."
+                } else {
+                    alert.messageText = "Margin could not be set as the default Markdown editor."
+                    alert.informativeText = "You can set it manually: choose a .md file in Finder, "
+                        + "press Command-I, and select Margin as the default app."
+                }
+                alert.runModal()
+            }
+        }
+    }
+
     @objc func newWindow(_ sender: Any?) {
         makeWorkspaceWindow(for: nil)
     }
diff --git a/Sources/MarginApp/AppMenu.swift b/Sources/MarginApp/AppMenu.swift
index b4039a7..37f703e 100644
--- a/Sources/MarginApp/AppMenu.swift
+++ b/Sources/MarginApp/AppMenu.swift
@@ -151,6 +151,14 @@ enum AppMenu {
         )
         fileMenu.addItem(.separator())
         fileMenu.addItem(item("Close Tab", action: #selector(NSWindow.performClose(_:)), key: "w"))
+        fileMenu.addItem(.separator())
+        fileMenu.addItem(
+            item(
+                "Set as Default Markdown Editor",
+                action: #selector(AppDelegate.setAsDefaultMarkdownEditor(_:)),
+                target: delegate
+            )
+        )
         mainMenu.addItem(menuItem(title: "File", submenu: fileMenu))
 
         let editMenu = NSMenu(title: "Edit")
diff --git a/Tests/MarginAppTests/WorkspaceBehaviorTests.swift b/Tests/MarginAppTests/WorkspaceBehaviorTests.swift
index 449b33f..d8b84b0 100644
--- a/Tests/MarginAppTests/WorkspaceBehaviorTests.swift
+++ b/Tests/MarginAppTests/WorkspaceBehaviorTests.swift
@@ -224,6 +224,16 @@ final class WorkspaceBehaviorTests: XCTestCase {
         )
     }
 
+    func testFileMenuOffersDefaultMarkdownEditorRegistration() {
+        let delegate = AppDelegate()
+        AppMenu.install(for: NSApplication.shared, delegate: delegate)
+
+        let item = menuItem(named: "Set as Default Markdown Editor")
+        XCTAssertNotNil(item)
+        XCTAssertEqual(item?.action, #selector(AppDelegate.setAsDefaultMarkdownEditor(_:)))
+        XCTAssertTrue(item?.target === delegate)
+    }
+
     func testFileWatcherNeverBlocksTheInteractionThreadWhileOpening() {
         let openerFinished = expectation(description: "Slow descriptor open finished")
         let watcher = FileSystemWatcher(