Abstract: An exhaust muffler for mounting on the transom of a powerboat in association with the exhaust pipes which extend through the transom. The muffler comprises a housing extending laterally across the stern of the boat and rearwardly from the transom to define an interior space. The housing has a floor located below the water line; opposed end walls; a front wall located adjacent to the transom with openings formed therein, through which exhaust gas can pass from the exhaust pipes; a rear wall having exhaust ports formed therein and a cover that encloses the interior space. Located within the enclosed space are muffler inserts secured to the floor and also to the front and rear walls. Each insert has a domed portion defining a muffling chamber that communicates with the respective exhaust pipe, and a second domed chamber defining an expansion chamber that communicates with the respective exhaust port located in the rear wall of the housing.