To provide a device for preventing water intrusion to a marine engine exhaust system capable of positively preventing intrusion of water to the exhaust system during engine stoppage without causing expansion of the engine exhaust system.

An exhaust valve 52 opening and closing an exhaust pipe 24 is provided in midway of the exhaust pipe 24 of a marine engine 20 provided with the exhaust pipe 24 opened underwater. A pressure deriving port 69 is opened in a side wall of an engine cylinder 26. The exhaust valve 52 is closed during engine stoppage, and it is opened by a positive pressure derived from the pressure deriving port 69 concurrently with an engine start. Since the exhaust pipe 24 is opened in association with start/stoppage of the engine by using the positive pressure generated in the engine cylinder 26, there is no need to provide an inverted U-shaped high riser in a part of the exhaust pipe 24, and intrusion of water to the exhaust system during engine stoppage can be positively prevented without causing expansion of the engine exhaust system.