diff --git a/doc/includes/references.h b/doc/includes/references.h
index d59a397b..4e4f282a 100644
--- a/doc/includes/references.h
+++ b/doc/includes/references.h
@@ -312,6 +312,10 @@ struct mjData_ {
   int*    B_rownnz;          // body-dof: non-zeros in each row                  (nbody x 1)
   int*    B_rowadr;          // body-dof: address of each row in B_colind        (nbody x 1)
   int*    B_colind;          // body-dof: column indices of non-zeros            (nB x 1)
+  int*    M_rownnz;          // inertia: non-zeros in each row                   (nv x 1)
+  int*    M_rowadr;          // inertia: address of each row in M_colind         (nv x 1)
+  int*    M_colind;          // inertia: column indices of non-zeros             (nM x 1)
+  int*    mapM2M;            // index mapping from M (legacy) to M (CSR)         (nM x 1)
   int*    C_rownnz;          // reduced dof-dof: non-zeros in each row           (nv x 1)
   int*    C_rowadr;          // reduced dof-dof: address of each row in C_colind (nv x 1)
   int*    C_colind;          // reduced dof-dof: column indices of non-zeros     (nC x 1)
diff --git a/include/mujoco/mjdata.h b/include/mujoco/mjdata.h
index c573a2c7..d69a1d4e 100644
--- a/include/mujoco/mjdata.h
+++ b/include/mujoco/mjdata.h
@@ -340,6 +340,10 @@ struct mjData_ {
   int*    B_rownnz;          // body-dof: non-zeros in each row                  (nbody x 1)
   int*    B_rowadr;          // body-dof: address of each row in B_colind        (nbody x 1)
   int*    B_colind;          // body-dof: column indices of non-zeros            (nB x 1)
+  int*    M_rownnz;          // inertia: non-zeros in each row                   (nv x 1)
+  int*    M_rowadr;          // inertia: address of each row in M_colind         (nv x 1)
+  int*    M_colind;          // inertia: column indices of non-zeros             (nM x 1)
+  int*    mapM2M;            // index mapping from M (legacy) to M (CSR)         (nM x 1)
   int*    C_rownnz;          // reduced dof-dof: non-zeros in each row           (nv x 1)
   int*    C_rowadr;          // reduced dof-dof: address of each row in C_colind (nv x 1)
   int*    C_colind;          // reduced dof-dof: column indices of non-zeros     (nC x 1)
diff --git a/include/mujoco/mjxmacro.h b/include/mujoco/mjxmacro.h
index a86dca46..2c21aaea 100644
--- a/include/mujoco/mjxmacro.h
+++ b/include/mujoco/mjxmacro.h
@@ -672,6 +672,10 @@
     X   ( int,       B_rownnz,          nbody,       1           ) \
     X   ( int,       B_rowadr,          nbody,       1           ) \
     X   ( int,       B_colind,          nB,          1           ) \
+    X   ( int,       M_rownnz,          nv,          1           ) \
+    X   ( int,       M_rowadr,          nv,          1           ) \
+    X   ( int,       M_colind,          nM,          1           ) \
+    X   ( int,       mapM2M,            nM,          1           ) \
     X   ( int,       C_rownnz,          nv,          1           ) \
     X   ( int,       C_rowadr,          nv,          1           ) \
     X   ( int,       C_colind,          nC,          1           ) \
diff --git a/mjx/mujoco/mjx/_src/io.py b/mjx/mujoco/mjx/_src/io.py
index 085d2534..a054af78 100644
--- a/mjx/mujoco/mjx/_src/io.py
+++ b/mjx/mujoco/mjx/_src/io.py
@@ -339,6 +339,10 @@ def make_data(
         'B_rownnz': (m.nbody, jp.int32),
         'B_rowadr': (m.nbody, jp.int32),
         'B_colind': (m.nB, jp.int32),
+        'M_rownnz': (m.nv, jp.int32),
+        'M_rowadr': (m.nv, jp.int32),
+        'M_colind': (m.nM, jp.int32),
+        'mapM2M': (m.nM, jp.int32),
         'C_rownnz': (m.nv, jp.int32),
         'C_rowadr': (m.nv, jp.int32),
         'C_colind': (m.nC, jp.int32),
diff --git a/mjx/mujoco/mjx/_src/types.py b/mjx/mujoco/mjx/_src/types.py
index 9a9d2146..b8945fff 100644
--- a/mjx/mujoco/mjx/_src/types.py
+++ b/mjx/mujoco/mjx/_src/types.py
@@ -1316,6 +1316,10 @@ class Data(PyTreeNode):
     B_rownnz: body-dof: non-zeros in each row                   (nbody,)
     B_rowadr: body-dof: address of each row in B_colind         (nbody,)
     B_colind: body-dof: column indices of non-zeros             (nB,)
+    M_rownnz: inertia: non-zeros in each row                    (nv,)
+    M_rowadr: inertia: address of each row in M_colind          (nv,)
+    M_colind: inertia: column indices of non-zeros              (nM,)
+    mapM2M: index mapping from M (legacy) to M (CSR)            (nM,)
     C_rownnz: reduced dof-dof: non-zeros in each row            (nv,)
     C_rowadr: reduced dof-dof: address of each row in C_colind  (nv,)
     C_colind: reduced dof-dof: column indices of non-zeros      (nC,)
@@ -1445,6 +1449,10 @@ class Data(PyTreeNode):
   B_rownnz: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
   B_rowadr: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
   B_colind: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
+  M_rownnz: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
+  M_rowadr: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
+  M_colind: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
+  mapM2M: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
   C_rownnz: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
   C_rowadr: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
   C_colind: jax.Array = _restricted_to('mujoco')  # pylint:disable=invalid-name
diff --git a/python/mujoco/introspect/structs.py b/python/mujoco/introspect/structs.py
index 41c94673..9a98b07c 100644
--- a/python/mujoco/introspect/structs.py
+++ b/python/mujoco/introspect/structs.py
@@ -5526,6 +5526,38 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='body-dof: column indices of non-zeros',
                  array_extent=('nB',),
              ),
+             StructFieldDecl(
+                 name='M_rownnz',
+                 type=PointerType(
+                     inner_type=ValueType(name='int'),
+                 ),
+                 doc='inertia: non-zeros in each row',
+                 array_extent=('nv',),
+             ),
+             StructFieldDecl(
+                 name='M_rowadr',
+                 type=PointerType(
+                     inner_type=ValueType(name='int'),
+                 ),
+                 doc='inertia: address of each row in M_colind',
+                 array_extent=('nv',),
+             ),
+             StructFieldDecl(
+                 name='M_colind',
+                 type=PointerType(
+                     inner_type=ValueType(name='int'),
+                 ),
+                 doc='inertia: column indices of non-zeros',
+                 array_extent=('nM',),
+             ),
+             StructFieldDecl(
+                 name='mapM2M',
+                 type=PointerType(
+                     inner_type=ValueType(name='int'),
+                 ),
+                 doc='index mapping from M (legacy) to M (CSR)',
+                 array_extent=('nM',),
+             ),
              StructFieldDecl(
                  name='C_rownnz',
                  type=PointerType(
diff --git a/unity/Runtime/Bindings/MjBindings.cs b/unity/Runtime/Bindings/MjBindings.cs
index b152a711..c529224c 100644
--- a/unity/Runtime/Bindings/MjBindings.cs
+++ b/unity/Runtime/Bindings/MjBindings.cs
@@ -4942,6 +4942,10 @@ public unsafe struct mjData_ {
   public int* B_rownnz;
   public int* B_rowadr;
   public int* B_colind;
+  public int* M_rownnz;
+  public int* M_rowadr;
+  public int* M_colind;
+  public int* mapM2M;
   public int* C_rownnz;
   public int* C_rowadr;
   public int* C_colind;