diff --git a/Cargo.lock b/Cargo.lock
index a15525ddde..9458e1fd8d 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1680,6 +1680,7 @@ dependencies = [
  "criterion",
  "crossterm",
  "dirs",
+ "dunce",
  "esl01-renderdag",
  "futures 0.3.30",
  "git2",
diff --git a/Cargo.toml b/Cargo.toml
index 4041e65b7b..eb73a1eee0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -44,6 +44,7 @@ criterion = "0.5.1"
 crossterm = { version = "0.27", default-features = false }
 digest = "0.10.7"
 dirs = "5.0.1"
+dunce = "1.0.4"
 either = "1.13.0"
 esl01-renderdag = "0.3.0"
 futures = "0.3.30"
diff --git a/cli/Cargo.toml b/cli/Cargo.toml
index fd1085b8b9..4204887da5 100644
--- a/cli/Cargo.toml
+++ b/cli/Cargo.toml
@@ -61,6 +61,7 @@ config = { workspace = true }
 criterion = { workspace = true, optional = true }
 crossterm = { workspace = true }
 dirs = { workspace = true }
+dunce = { workspace = true }
 esl01-renderdag = { workspace = true }
 futures = { workspace = true }
 git2 = { workspace = true }
diff --git a/cli/src/cli_util.rs b/cli/src/cli_util.rs
index d33142d5da..af025144bd 100644
--- a/cli/src/cli_util.rs
+++ b/cli/src/cli_util.rs
@@ -2138,6 +2138,9 @@ pub fn get_new_config_file_path(
 }
 
 pub fn run_ui_editor(settings: &UserSettings, edit_path: &PathBuf) -> Result<(), CommandError> {
+    // Work around UNC paths not being well supported on Windows (no-op for non-Windows):
+    // https://github.com/martinvonz/jj/issues/3986
+    let edit_path = dunce::simplified(edit_path);
     let editor: CommandNameAndArgs = settings
         .config()
         .get("ui.editor")
diff --git a/cli/tests/test_commit_command.rs b/cli/tests/test_commit_command.rs
index 5d706de574..d29b598169 100644
--- a/cli/tests/test_commit_command.rs
+++ b/cli/tests/test_commit_command.rs
@@ -73,6 +73,28 @@ fn test_commit_with_editor() {
     "###);
 }
 
+#[test]
+#[cfg(windows)]
+fn test_commit_with_editor_avoids_unc() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let workspace_path = test_env.env_root().join("repo");
+    let edit_script = test_env.set_up_fake_editor();
+
+    std::fs::write(edit_script, ["dumppath path"].join("\0")).unwrap();
+    test_env.jj_cmd_ok(&workspace_path, &["commit"]);
+
+    let editor_path = std::fs::read_to_string(test_env.env_root().join("path")).unwrap();
+    // While `assert!(!editor_path.starts_with("//?/"))` could work here in most cases,
+    // it fails when it is not safe to strip the prefix, such as paths over 260 chars.
+    assert_eq!(
+        editor_path,
+        dunce::simplified(std::path::Path::new(&editor_path))
+            .to_str()
+            .unwrap()
+    );
+}
+
 #[test]
 fn test_commit_interactive() {
     let mut test_env = TestEnvironment::default();
diff --git a/cli/tests/test_config_command.rs b/cli/tests/test_config_command.rs
index 75b5bb200a..ec2d64eb59 100644
--- a/cli/tests/test_config_command.rs
+++ b/cli/tests/test_config_command.rs
@@ -626,7 +626,7 @@ fn test_config_edit_user() {
 
     assert_eq!(
         std::fs::read_to_string(test_env.env_root().join("path")).unwrap(),
-        test_env.config_path().to_str().unwrap()
+        dunce::simplified(test_env.config_path()).to_str().unwrap()
     );
 }
 
@@ -642,7 +642,9 @@ fn test_config_edit_repo() {
 
     assert_eq!(
         std::fs::read_to_string(test_env.env_root().join("path")).unwrap(),
-        repo_path.join(".jj/repo/config.toml").to_str().unwrap()
+        dunce::simplified(&repo_path.join(".jj/repo/config.toml"))
+            .to_str()
+            .unwrap()
     );
 }
 
diff --git a/cli/tests/test_describe_command.rs b/cli/tests/test_describe_command.rs
index fc88ad4b5b..c0a80e5956 100644
--- a/cli/tests/test_describe_command.rs
+++ b/cli/tests/test_describe_command.rs
@@ -323,3 +323,25 @@ fn test_describe_author() {
     ~
     "###);
 }
+
+#[test]
+#[cfg(windows)]
+fn test_describe_avoids_unc() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let workspace_path = test_env.env_root().join("repo");
+    let edit_script = test_env.set_up_fake_editor();
+
+    std::fs::write(edit_script, "dumppath path").unwrap();
+    test_env.jj_cmd_ok(&workspace_path, &["describe"]);
+
+    let editor_path = std::fs::read_to_string(test_env.env_root().join("path")).unwrap();
+    // While `assert!(!editor_path.starts_with("//?/"))` could work here in most cases,
+    // it fails when it is not safe to strip the prefix, such as paths over 260 chars.
+    assert_eq!(
+        editor_path,
+        dunce::simplified(std::path::Path::new(&editor_path))
+            .to_str()
+            .unwrap()
+    );
+}
diff --git a/cli/tests/test_move_command.rs b/cli/tests/test_move_command.rs
index 41a38be861..8b69c0c3e4 100644
--- a/cli/tests/test_move_command.rs
+++ b/cli/tests/test_move_command.rs
@@ -432,6 +432,36 @@ fn test_move_description() {
     "###);
 }
 
+#[test]
+#[cfg(windows)]
+fn test_move_description_editor_avoids_unc() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    let edit_script = test_env.set_up_fake_editor();
+    std::fs::write(repo_path.join("file1"), "a\n").unwrap();
+    std::fs::write(repo_path.join("file2"), "a\n").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["new"]);
+    std::fs::write(repo_path.join("file1"), "b\n").unwrap();
+    std::fs::write(repo_path.join("file2"), "b\n").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["describe", "@-", "-m", "destination"]);
+    test_env.jj_cmd_ok(&repo_path, &["describe", "-m", "source"]);
+
+    std::fs::write(&edit_script, "dumppath path").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["move", "--to", "@-"]);
+
+    let editor_path = std::fs::read_to_string(test_env.env_root().join("path")).unwrap();
+    // While `assert!(!editor_path.starts_with("//?/"))` could work here in most cases,
+    // it fails when it is not safe to strip the prefix, such as paths over 260 chars.
+    assert_eq!(
+        editor_path,
+        dunce::simplified(std::path::Path::new(&editor_path))
+            .to_str()
+            .unwrap()
+    );
+}
+
 fn get_description(test_env: &TestEnvironment, repo_path: &Path, rev: &str) -> String {
     test_env.jj_cmd_success(
         repo_path,
diff --git a/cli/tests/test_sparse_command.rs b/cli/tests/test_sparse_command.rs
index b01e791342..03edf2d5f6 100644
--- a/cli/tests/test_sparse_command.rs
+++ b/cli/tests/test_sparse_command.rs
@@ -174,3 +174,24 @@ fn test_sparse_manage_patterns() {
     file3
     "###);
 }
+
+#[test]
+fn test_sparse_editor_avoids_unc() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+    let edit_script = test_env.set_up_fake_editor();
+
+    std::fs::write(&edit_script, "dumppath path").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["sparse", "edit"]);
+
+    let editor_path = std::fs::read_to_string(test_env.env_root().join("path")).unwrap();
+    // While `assert!(!editor_path.starts_with("//?/"))` could work here in most cases,
+    // it fails when it is not safe to strip the prefix, such as paths over 260 chars.
+    assert_eq!(
+        editor_path,
+        dunce::simplified(std::path::Path::new(&editor_path))
+            .to_str()
+            .unwrap()
+    );
+}
diff --git a/cli/tests/test_split_command.rs b/cli/tests/test_split_command.rs
index 83b244daa8..0adb78c219 100644
--- a/cli/tests/test_split_command.rs
+++ b/cli/tests/test_split_command.rs
@@ -538,3 +538,28 @@ fn test_split_empty() {
     Hint: Use `jj new` if you want to create another empty commit.
     "###);
 }
+
+#[test]
+#[cfg(windows)]
+fn test_split_message_editor_avoids_unc() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    std::fs::write(repo_path.join("file1"), "foo").unwrap();
+    std::fs::write(repo_path.join("file2"), "foo").unwrap();
+
+    let edit_script = test_env.set_up_fake_editor();
+    std::fs::write(edit_script, "dumppath path").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["split", "file2"]);
+
+    let editor_path = std::fs::read_to_string(test_env.env_root().join("path")).unwrap();
+    // While `assert!(!editor_path.starts_with("//?/"))` could work here in most cases,
+    // it fails when it is not safe to strip the prefix, such as paths over 260 chars.
+    assert_eq!(
+        editor_path,
+        dunce::simplified(std::path::Path::new(&editor_path))
+            .to_str()
+            .unwrap()
+    );
+}
diff --git a/cli/tests/test_squash_command.rs b/cli/tests/test_squash_command.rs
index b3f8d1ed41..d1a0445473 100644
--- a/cli/tests/test_squash_command.rs
+++ b/cli/tests/test_squash_command.rs
@@ -1030,6 +1030,36 @@ fn test_squash_description() {
     "###);
 }
 
+#[test]
+#[cfg(windows)]
+fn test_squash_description_editor_avoids_unc() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    let edit_script = test_env.set_up_fake_editor();
+    std::fs::write(repo_path.join("file1"), "a\n").unwrap();
+    std::fs::write(repo_path.join("file2"), "a\n").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["new"]);
+    std::fs::write(repo_path.join("file1"), "b\n").unwrap();
+    std::fs::write(repo_path.join("file2"), "b\n").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["describe", "@-", "-m", "destination"]);
+    test_env.jj_cmd_ok(&repo_path, &["describe", "-m", "source"]);
+
+    std::fs::write(&edit_script, "dumppath path").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["squash"]);
+
+    let editor_path = std::fs::read_to_string(test_env.env_root().join("path")).unwrap();
+    // While `assert!(!editor_path.starts_with("//?/"))` could work here in most cases,
+    // it fails when it is not safe to strip the prefix, such as paths over 260 chars.
+    assert_eq!(
+        editor_path,
+        dunce::simplified(std::path::Path::new(&editor_path))
+            .to_str()
+            .unwrap()
+    );
+}
+
 #[test]
 fn test_squash_empty() {
     let mut test_env = TestEnvironment::default();
diff --git a/cli/tests/test_unsquash_command.rs b/cli/tests/test_unsquash_command.rs
index dfdce7a43c..fe4cb28d7f 100644
--- a/cli/tests/test_unsquash_command.rs
+++ b/cli/tests/test_unsquash_command.rs
@@ -313,6 +313,36 @@ fn test_unsquash_description() {
     "###);
 }
 
+#[test]
+#[cfg(windows)]
+fn test_unsquash_description_editor_avoids_unc() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    let edit_script = test_env.set_up_fake_editor();
+    std::fs::write(repo_path.join("file1"), "a\n").unwrap();
+    std::fs::write(repo_path.join("file2"), "a\n").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["new"]);
+    std::fs::write(repo_path.join("file1"), "b\n").unwrap();
+    std::fs::write(repo_path.join("file2"), "b\n").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["describe", "@-", "-m", "destination"]);
+    test_env.jj_cmd_ok(&repo_path, &["describe", "-m", "source"]);
+
+    std::fs::write(&edit_script, "dumppath path").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["unsquash"]);
+
+    let editor_path = std::fs::read_to_string(test_env.env_root().join("path")).unwrap();
+    // While `assert!(!editor_path.starts_with("//?/"))` could work here in most cases,
+    // it fails when it is not safe to strip the prefix, such as paths over 260 chars.
+    assert_eq!(
+        editor_path,
+        dunce::simplified(std::path::Path::new(&editor_path))
+            .to_str()
+            .unwrap()
+    );
+}
+
 fn get_description(test_env: &TestEnvironment, repo_path: &Path, rev: &str) -> String {
     test_env.jj_cmd_success(
         repo_path,