diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1f6007fcf..122dd4dbd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 - Fix crash when removing a multibody joint, or a rigid-body with a multipody-joint attached to it.
 - Fix crash when inserting multibody joints in an arbitrary order (instead of incrementally from root to leaf).
+- Fix `BroadphaseMultiSap` not being able to serialize correctly with serde_json.
 
 ### Added
 
@@ -14,6 +15,7 @@
 
 - Divided by two the value of each `QueryFilterFlags` variant so that
   the smallest one is 1 instead of 2 (fixes a bug in rapier.js).
+- `BroadphaseMultiSap` now serializes its `colliders_proxy_ids` as `Vec[(ColliderHandle, BroadPhaseProxyIndex)]`.
 
 ## v0.21.0 (23 June 2024)
 
diff --git a/crates/rapier2d-f64/Cargo.toml b/crates/rapier2d-f64/Cargo.toml
index 2606f7d23..e5347a5d4 100644
--- a/crates/rapier2d-f64/Cargo.toml
+++ b/crates/rapier2d-f64/Cargo.toml
@@ -7,7 +7,13 @@ documentation = "https://docs.rs/rapier2d"
 homepage = "https://rapier.rs"
 repository = "https://github.com/dimforge/rapier"
 readme = "README.md"
-categories = ["science", "game-development", "mathematics", "simulation", "wasm"]
+categories = [
+    "science",
+    "game-development",
+    "mathematics",
+    "simulation",
+    "wasm",
+]
 keywords = ["physics", "dynamics", "rigid", "real-time", "impulse_joints"]
 license = "Apache-2.0"
 edition = "2021"
@@ -26,7 +32,13 @@ simd-nightly = ["simba/portable_simd", "simd-is-enabled"]
 # enabled with the "simd-stable" or "simd-nightly" feature.
 simd-is-enabled = ["dep:vec_map"]
 wasm-bindgen = ["instant/wasm-bindgen"]
-serde-serialize = ["nalgebra/serde-serialize", "parry2d-f64/serde-serialize", "dep:serde", "bit-vec/serde", "arrayvec/serde"]
+serde-serialize = [
+    "nalgebra/serde-serialize",
+    "parry2d-f64/serde-serialize",
+    "dep:serde",
+    "bit-vec/serde",
+    "arrayvec/serde",
+]
 enhanced-determinism = ["simba/libm_force", "parry2d-f64/enhanced-determinism"]
 debug-render = []
 profiler = ["dep:instant"] # Enables the internal profiler.
@@ -70,5 +82,6 @@ thiserror = "1"
 
 [dev-dependencies]
 bincode = "1"
+serde_json = "1"
 serde = { version = "1", features = ["derive"] }
 oorandom = { version = "11", default-features = false }
diff --git a/crates/rapier2d/Cargo.toml b/crates/rapier2d/Cargo.toml
index ec363c1fe..e8e1ad26a 100644
--- a/crates/rapier2d/Cargo.toml
+++ b/crates/rapier2d/Cargo.toml
@@ -7,7 +7,13 @@ documentation = "https://docs.rs/rapier2d"
 homepage = "https://rapier.rs"
 repository = "https://github.com/dimforge/rapier"
 readme = "README.md"
-categories = ["science", "game-development", "mathematics", "simulation", "wasm"]
+categories = [
+    "science",
+    "game-development",
+    "mathematics",
+    "simulation",
+    "wasm",
+]
 keywords = ["physics", "dynamics", "rigid", "real-time", "impulse_joints"]
 license = "Apache-2.0"
 edition = "2021"
@@ -26,7 +32,13 @@ simd-nightly = ["simba/portable_simd", "simd-is-enabled"]
 # enabled with the "simd-stable" or "simd-nightly" feature.
 simd-is-enabled = ["dep:vec_map"]
 wasm-bindgen = ["instant/wasm-bindgen"]
-serde-serialize = ["nalgebra/serde-serialize", "parry2d/serde-serialize", "dep:serde", "bit-vec/serde", "arrayvec/serde"]
+serde-serialize = [
+    "nalgebra/serde-serialize",
+    "parry2d/serde-serialize",
+    "dep:serde",
+    "bit-vec/serde",
+    "arrayvec/serde",
+]
 enhanced-determinism = ["simba/libm_force", "parry2d/enhanced-determinism"]
 debug-render = []
 profiler = ["dep:instant"] # Enables the internal profiler.
@@ -70,5 +82,6 @@ thiserror = "1"
 
 [dev-dependencies]
 bincode = "1"
+serde_json = "1"
 serde = { version = "1", features = ["derive"] }
-oorandom = { version = "11", default-features = false }
\ No newline at end of file
+oorandom = { version = "11", default-features = false }
diff --git a/crates/rapier3d-f64/Cargo.toml b/crates/rapier3d-f64/Cargo.toml
index 17390e73f..c1d9ce7e2 100644
--- a/crates/rapier3d-f64/Cargo.toml
+++ b/crates/rapier3d-f64/Cargo.toml
@@ -7,7 +7,13 @@ documentation = "https://docs.rs/rapier3d"
 homepage = "https://rapier.rs"
 repository = "https://github.com/dimforge/rapier"
 readme = "README.md"
-categories = ["science", "game-development", "mathematics", "simulation", "wasm"]
+categories = [
+    "science",
+    "game-development",
+    "mathematics",
+    "simulation",
+    "wasm",
+]
 keywords = ["physics", "dynamics", "rigid", "real-time", "impulse_joints"]
 license = "Apache-2.0"
 edition = "2021"
@@ -21,12 +27,21 @@ dim3 = []
 f64 = []
 parallel = ["dep:rayon"]
 simd-stable = ["parry3d-f64/simd-stable", "simba/wide", "simd-is-enabled"]
-simd-nightly = ["parry3d-f64/simd-nightly", "simba/portable_simd", "simd-is-enabled"]
+simd-nightly = [
+    "parry3d-f64/simd-nightly",
+    "simba/portable_simd",
+    "simd-is-enabled",
+]
 # Do not enable this feature directly. It is automatically
 # enabled with the "simd-stable" or "simd-nightly" feature.
 simd-is-enabled = ["dep:vec_map"]
 wasm-bindgen = ["instant/wasm-bindgen"]
-serde-serialize = ["nalgebra/serde-serialize", "parry3d-f64/serde-serialize", "dep:serde", "bit-vec/serde"]
+serde-serialize = [
+    "nalgebra/serde-serialize",
+    "parry3d-f64/serde-serialize",
+    "dep:serde",
+    "bit-vec/serde",
+]
 enhanced-determinism = ["simba/libm_force", "parry3d-f64/enhanced-determinism"]
 debug-render = []
 profiler = ["dep:instant"] # Enables the internal profiler.
@@ -70,5 +85,6 @@ thiserror = "1"
 
 [dev-dependencies]
 bincode = "1"
+serde_json = "1"
 serde = { version = "1", features = ["derive"] }
 oorandom = { version = "11", default-features = false }
diff --git a/crates/rapier3d/Cargo.toml b/crates/rapier3d/Cargo.toml
index 3f45d7bad..03947ef9b 100644
--- a/crates/rapier3d/Cargo.toml
+++ b/crates/rapier3d/Cargo.toml
@@ -7,7 +7,13 @@ documentation = "https://docs.rs/rapier3d"
 homepage = "https://rapier.rs"
 repository = "https://github.com/dimforge/rapier"
 readme = "README.md"
-categories = ["science", "game-development", "mathematics", "simulation", "wasm"]
+categories = [
+    "science",
+    "game-development",
+    "mathematics",
+    "simulation",
+    "wasm",
+]
 keywords = ["physics", "dynamics", "rigid", "real-time", "impulse_joints"]
 license = "Apache-2.0"
 edition = "2021"
@@ -21,12 +27,21 @@ dim3 = []
 f32 = []
 parallel = ["dep:rayon"]
 simd-stable = ["parry3d/simd-stable", "simba/wide", "simd-is-enabled"]
-simd-nightly = ["parry3d/simd-nightly", "simba/portable_simd", "simd-is-enabled"]
+simd-nightly = [
+    "parry3d/simd-nightly",
+    "simba/portable_simd",
+    "simd-is-enabled",
+]
 # Do not enable this feature directly. It is automatically
 # enabled with the "simd-stable" or "simd-nightly" feature.
 simd-is-enabled = ["dep:vec_map"]
 wasm-bindgen = ["instant/wasm-bindgen"]
-serde-serialize = ["nalgebra/serde-serialize", "parry3d/serde-serialize", "dep:serde", "bit-vec/serde"]
+serde-serialize = [
+    "nalgebra/serde-serialize",
+    "parry3d/serde-serialize",
+    "dep:serde",
+    "bit-vec/serde",
+]
 enhanced-determinism = ["simba/libm_force", "parry3d/enhanced-determinism"]
 debug-render = []
 profiler = ["dep:instant"] # Enables the internal profiler.
@@ -70,5 +85,6 @@ thiserror = "1"
 
 [dev-dependencies]
 bincode = "1"
+serde_json = "1"
 serde = { version = "1", features = ["derive"] }
 oorandom = { version = "11", default-features = false }
diff --git a/src/geometry/broad_phase_multi_sap/broad_phase_multi_sap.rs b/src/geometry/broad_phase_multi_sap/broad_phase_multi_sap.rs
index 87b2269f3..1d0f2a4ce 100644
--- a/src/geometry/broad_phase_multi_sap/broad_phase_multi_sap.rs
+++ b/src/geometry/broad_phase_multi_sap/broad_phase_multi_sap.rs
@@ -90,6 +90,13 @@ pub struct BroadPhaseMultiSap {
     //       Another alternative would be to remove ColliderProxyId and
     //       just use a Coarena. But this seems like it could use too
     //       much memory.
+    #[cfg_attr(
+        feature = "serde-serialize",
+        serde(
+            serialize_with = "crate::utils::serde::serialize_to_vec_tuple",
+            deserialize_with = "crate::utils::serde::deserialize_from_vec_tuple"
+        )
+    )]
     colliders_proxy_ids: HashMap<ColliderHandle, BroadPhaseProxyIndex>,
     #[cfg_attr(feature = "serde-serialize", serde(skip))]
     region_pool: SAPRegionPool, // To avoid repeated allocations.
diff --git a/src/utils.rs b/src/utils.rs
index 102c85f2d..84acb21de 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -667,3 +667,75 @@ pub fn smallest_abs_diff_between_angles<N: SimdRealCopy>(a: N, b: N) -> N {
     let s_err_is_smallest = s_err.simd_abs().simd_lt(s_err_complement.simd_abs());
     s_err.select(s_err_is_smallest, s_err_complement)
 }
+
+/// Helpers around serialization.
+#[cfg(feature = "serde-serialize")]
+pub mod serde {
+    use serde::{Deserialize, Serialize};
+    use std::iter::FromIterator;
+
+    /// Serializes to a `Vec<(K, V)>`.
+    ///
+    /// Useful for [`std::collections::HashMap`] with a non-string key,
+    /// which is unsupported by [`serde_json`].
+    pub fn serialize_to_vec_tuple<
+        'a,
+        S: serde::Serializer,
+        T: IntoIterator<Item = (&'a K, &'a V)>,
+        K: Serialize + 'a,
+        V: Serialize + 'a,
+    >(
+        target: T,
+        s: S,
+    ) -> Result<S::Ok, S::Error> {
+        let container: Vec<_> = target.into_iter().collect();
+        serde::Serialize::serialize(&container, s)
+    }
+
+    /// Deserializes from a `Vec<(K, V)>`.
+    ///
+    /// Useful for [`std::collections::HashMap`] with a non-string key,
+    /// which is unsupported by [`serde_json`].
+    pub fn deserialize_from_vec_tuple<
+        'de,
+        D: serde::Deserializer<'de>,
+        T: FromIterator<(K, V)>,
+        K: Deserialize<'de>,
+        V: Deserialize<'de>,
+    >(
+        d: D,
+    ) -> Result<T, D::Error> {
+        let hashmap_as_vec: Vec<(K, V)> = Deserialize::deserialize(d)?;
+        Ok(T::from_iter(hashmap_as_vec))
+    }
+
+    #[cfg(test)]
+    mod test {
+        use std::collections::HashMap;
+
+        /// This test uses serde_json because json doesn't support non string
+        /// keys in hashmaps, which requires a custom serialization.
+        #[test]
+        fn serde_json_hashmap() {
+            #[derive(Serialize, Deserialize, PartialEq, Eq, Debug)]
+            struct Test {
+                #[cfg_attr(
+                    feature = "serde-serialize",
+                    serde(
+                        serialize_with = "crate::utils::serde::serialize_to_vec_tuple",
+                        deserialize_with = "crate::utils::serde::deserialize_from_vec_tuple"
+                    )
+                )]
+                pub map: HashMap<usize, String>,
+            }
+
+            let s = Test {
+                map: [(42, "Forty-Two".to_string())].into(),
+            };
+            let j = serde_json::to_string(&s).unwrap();
+            assert_eq!(&j, "{\"map\":[[42,\"Forty-Two\"]]}");
+            let p: Test = serde_json::from_str(&j).unwrap();
+            assert_eq!(&p, &s);
+        }
+    }
+}