According to Devon and Cornwall Police, the incident occurred at the public Toilets at The Leats, sometime between 4.30pm and 5pm on Thursday, December 14.
The victim, a man in his 30s was assaulted by another man and is now receiving support from specially trained officers.
A force spokesperson said: “Police are investigating a report of a serious sexual assault on a man in Truro yesterday, 14 December.
“The man was assaulted by another man within the public toilets at The Leats yesterday sometime between 4.30pm and 5pm.
“The victim, a man in his 30s, is being supported by specially trained officers.
“Detectives are appealing for anyone who saw anything suspicious in the area at the time or anyone who may have information to assist with enquiries to contact them.
“Please contact police online or by telephone on 101, quoting crime reference 50230324951.
“Information can also be passed anonymously to independent charity Crimestoppers via 0800 555111 or www.crimestoppers-uk.org.”