Putnam County Sheriff Donald Smith says the gun was found Wednesday near the riverbank off a state park in Cold Spring.
He says it's apparently the weapon used by Sang Ho Kim on Sept. 25 to shoot two men, killing one, at an import firm's office in Garden City.
Nassau County police said the shooting involved a business dispute.
Kim fled, and the search for him moved from Long Island to New York City's northern suburbs when his car was found at the park in Cold Spring.
Kim's body was found Sept. 30, 10 miles downstream in the Hudson. Police said he shot himself and drowned.