A muffler for mounting to a handheld pneumatic tool, such as a die grinder. The muffler comprises a hollow open-ended casing having an inner wall lined with a thick layer of an insulating material. The muffler is sleeve mounted about a coupler positioned between an exhaust end of the pneumatic tool and an air adapter supplying compressed air to the pneumatic tool for operation thereof. A plate comprising a plurality of perforations is positioned at a first face of the casing adjacent the air adapter. A biasing member is further placed between the plate and the air adapter for biasing a second face of the casing against the exhaust end. In this manner, during operation of the pneumatic tool, released air expelled at the exhaust end is directed through the casing to reduce a level of the noise at the exhaust end.