The 33-year-old victim left a wedding reception early in the morning of September 16 and was supposed to meet a group of friends at a bar in Pike Alley.
He never showed up, and was found unconscious in Belltown later that morning, according to police. Police say he was missing his wallet, watch and cell phone.
The victim was in and out of consciousness for the rest of the week, but his current condition is not known.
Detectives are investigating the case as a robbery, but are still trying to determine if the man was attacked or injured in some other way.
Police are now trying to track down two men and a woman who were seen on surveillance footage talking to the victim less than an hour before he was found.
Anyone with information about the trio is asked to call the police.