diff --git a/src/xrt/include/xrt/xrt_defines.h b/src/xrt/include/xrt/xrt_defines.h
index 922542418..dec46511c 100644
--- a/src/xrt/include/xrt/xrt_defines.h
+++ b/src/xrt/include/xrt/xrt_defines.h
@@ -246,6 +246,16 @@ struct xrt_matrix_2x2
 	};
 };
 
+/*!
+ * A tightly packed 3x3 matrix of floats.
+ *
+ * @ingroup xrt_iface math
+ */
+struct xrt_matrix_3x3
+{
+	float v[9];
+};
+
 /*!
  * A tightly packed 4x4 matrix of floats.
  *