diff --git a/src/xml/xml_native_writer.cc b/src/xml/xml_native_writer.cc
index 720c6132..2c35b11b 100644
--- a/src/xml/xml_native_writer.cc
+++ b/src/xml/xml_native_writer.cc
@@ -252,6 +252,7 @@ void mjXWriter::OneJoint(XMLElement* elem, mjCJoint* pjoint, mjCDef* def) {
 // write geom
 void mjXWriter::OneGeom(XMLElement* elem, mjCGeom* pgeom, mjCDef* def) {
   double unitq[4] = {1, 0, 0, 0};
+  double mass = 0;
 
   // regular
   if (!writingdefaults) {
@@ -260,6 +261,9 @@ void mjXWriter::OneGeom(XMLElement* elem, mjCGeom* pgeom, mjCDef* def) {
     if (mjGEOMINFO[pgeom->type]) {
       WriteAttr(elem, "size", mjGEOMINFO[pgeom->type], pgeom->size, def->geom.size);
     }
+    if (mjuu_defined(pgeom->_mass)) {
+      mass = pgeom->GetVolume() * def->geom.density;
+    }
 
     // mesh geom
     if (pgeom->type==mjGEOM_MESH) {
@@ -308,7 +312,6 @@ void mjXWriter::OneGeom(XMLElement* elem, mjCGeom* pgeom, mjCDef* def) {
   WriteAttr(elem, "fluidcoef", 5, pgeom->fluid_coefs, def->geom.fluid_coefs);
   WriteAttrKey(elem, "shellinertia", meshtype_map, 2, pgeom->typeinertia, def->geom.typeinertia);
   if (mjuu_defined(pgeom->_mass)) {
-    double mass = pgeom->GetVolume() * def->geom.density;
     WriteAttr(elem, "mass", 1, &pgeom->mass, &mass);
   } else {
     WriteAttr(elem, "density", 1, &pgeom->density, &def->geom.density);
diff --git a/test/xml/xml_native_writer_test.cc b/test/xml/xml_native_writer_test.cc
index bb4e650c..84c7722d 100644
--- a/test/xml/xml_native_writer_test.cc
+++ b/test/xml/xml_native_writer_test.cc
@@ -684,6 +684,31 @@ TEST_F(XMLWriterTest, OverwritesDensity) {
   mj_deleteModel(model);
 }
 
+TEST_F(XMLWriterTest, SaveDefaultMass) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <asset>
+      <mesh name="example"
+        vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+        face="2 0 3  0 1 3  1 2 3  0 2 1" />
+    </asset>
+    <default class="main">
+      <geom type="mesh" mass="1"/>
+    </default>
+    <worldbody>
+      <body>
+        <geom mesh="example" size=".1 .2 .3"/>
+      </body>
+    </worldbody>
+  </mujoco>
+  )";
+  mjModel* model = LoadModelFromString(xml);
+  EXPECT_THAT(model, NotNull());
+  std::string content = SaveAndReadXml(model);
+  EXPECT_THAT(content, HasSubstr("mass=\"1\""));
+  mj_deleteModel(model);
+}
+
 TEST_F(XMLWriterTest, UsesTwoSpaces) {
   static constexpr char xml[] = R"(
   <mujoco>