diff --git a/lib/src/extensions_map.rs b/lib/src/extensions_map.rs
new file mode 100644
index 000000000..7e9889a7f
--- /dev/null
+++ b/lib/src/extensions_map.rs
@@ -0,0 +1,98 @@
+// Copyright 2024 The Jujutsu Authors
+//
+// Licensed 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
+//
+// https://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.
+
+#![allow(missing_docs)]
+
+use std::any::{Any, TypeId};
+use std::collections::HashMap;
+
+/// Type-safe map that stores objects of arbitrary types.
+///
+/// This allows extensions to store and retrieve their own types unknown to
+/// jj_lib safely.
+#[derive(Default)]
+pub struct ExtensionsMap {
+    values: HashMap<TypeId, Box<dyn Any>>,
+}
+
+impl ExtensionsMap {
+    /// Creates an empty ExtensionsMap.
+    pub fn empty() -> Self {
+        Default::default()
+    }
+
+    /// Returns the specified type if it has already been inserted.
+    pub fn get<V: Any>(&self) -> Option<&V> {
+        self.values
+            .get(&TypeId::of::<V>())
+            .map(|v| v.downcast_ref::<V>().unwrap())
+    }
+
+    /// Inserts a new instance of the specified type.
+    ///
+    /// Requires that this type has not been inserted before.
+    pub fn insert<V: Any>(&mut self, value: V) {
+        assert!(self
+            .values
+            .insert(TypeId::of::<V>(), Box::new(value))
+            .is_none());
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    struct TestTypeA;
+    impl TestTypeA {
+        fn get_a(&self) -> &'static str {
+            "a"
+        }
+    }
+
+    struct TestTypeB;
+    impl TestTypeB {
+        fn get_b(&self) -> &'static str {
+            "b"
+        }
+    }
+
+    #[test]
+    fn test_empty() {
+        let extensions_map = ExtensionsMap::empty();
+        assert!(extensions_map.get::<TestTypeA>().is_none());
+        assert!(extensions_map.get::<TestTypeB>().is_none());
+    }
+
+    #[test]
+    fn test_retrieval() {
+        let mut extensions_map = ExtensionsMap::empty();
+        extensions_map.insert(TestTypeA);
+        extensions_map.insert(TestTypeB);
+        assert_eq!(
+            extensions_map
+                .get::<TestTypeA>()
+                .map(|a| a.get_a())
+                .unwrap_or(""),
+            "a"
+        );
+        assert_eq!(
+            extensions_map
+                .get::<TestTypeB>()
+                .map(|b| b.get_b())
+                .unwrap_or(""),
+            "b"
+        );
+    }
+}
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index 88b95c5ad..d09fb04a2 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -36,6 +36,7 @@ pub mod dag_walk;
 pub mod default_index;
 pub mod default_submodule_store;
 pub mod diff;
+pub mod extensions_map;
 pub mod file_util;
 pub mod files;
 pub mod fmt_util;