Lincolnshire police believe the man found in the River Witham to be 45-year-old Igors Petrovs.
Formal identification is still yet to take place.
Two men have been arrested in connection with the discovery of a body in a river in Lincoln earlier this week.
Officers were called to the Waterside North area of the city on Monday 24 August to reports of what appeared to be a person in the water.
Lincolnshire Police has now renewed its appeal for any information that may assist with its murder investigation.
A spokesperson said: "Following our initial enquiries, we will now be treating this as a murder investigation."We would like to renew our appeal and ask anyone who has information that might be relevant to our investigation to call 101.
"Two men, aged 41 and 43, have been arrested on suspicion of murder and remain in police custody."
Police say they have now identified the man whose body was found in the River Witham and have informed his next of kin.