diff --git a/third_party/flatbuffers/flatbuffer.gni b/third_party/flatbuffers/flatbuffer.gni
index 59b1a15304b09..9c6a1379af8e2 100644
--- a/third_party/flatbuffers/flatbuffer.gni
+++ b/third_party/flatbuffers/flatbuffer.gni
@@ -25,6 +25,10 @@
 #       fail it will try to load relative to the directory of the schema file
 #       being parsed.
 #
+#   mutable (optional)
+#       Boolean to compile flatbuffers with the "--gen-mutable" argument, which
+#       generates non-const accessors for mutating FlatBuffers in-place.
+#
 #   deps (optional)
 #       Additional dependencies.
 #
@@ -90,6 +94,10 @@ template("flatbuffer") {
       }
     }
 
+    if (defined(invoker.mutable) && invoker.mutable) {
+      args += [ "--gen-mutable" ]
+    }
+
     args += [ "{{source}}" ]
 
     # The deps may have steps that have to run before running flatc.