The dead man has been identified as a 31-year-old American, but his name has yet to be disclosed.
Police were alerted after locals reported a stench coming from a hostel room on Tran Nhan Tong Street, and subsequently discovered the decomposing body of a male foreigner.
Investigators are treating the death as a suicide.
“The victim had been staying at the hostel since 2016," Phap Luat (Law) newspaper quoted Colonel Dinh Xuan Nghi from Hoi An's police force as saying. "During that time, he wrote a suicide note and asked the landlord to send it to his relatives in the U.S.”
Further investigations are underway.
Earlier this year, an Australian tourist also committed suicide in Ha Long Bay. Stephen John Scott, 54, reportedly jumped off an anchored cruiser with his legs tied to a heavy dumbbell while visiting the popular tourism destination in February.