The little boy had been visiting the Calthorpe Project, which took place in central London, with his mother.
He then stepped into one of the community centre's toilets - and, when he returned, told his horrified mother that he had been sexually assaulted by a drunk man in the bathroom.
A staff member attempted to follow the suspect, who had an Irish accent and was believed to be drunk, but lost him in the street.
Police have now released CCTV footage in an attempt to track down the man:
Detective Inspector Neil Rawlinson, Sexual Offences, Exploitation and Child Abuse Command, said: "This was a particularly upsetting sexual assault on a young boy by a complete stranger.
"Numerous inquiries have been ongoing since the incident took place in an attempt to trace the suspect, who we believe was heavily intoxicated.
"Unfortunately those inquiries have not identified any individuals; the reason we have now taken the decision to go public given all other lines of inquiry have been exhausted.
"I would urge anyone who recognises the man to come forward and contact my team on 020 8733 5999. To remain anonymous call Crimestoppers on 0800 555 111.
"I would also like to praise the actions of the staff at the Calthorpe Project for their quick response to this matter and reassure the public that incidents of this nature are extremely rare."
The suspect is described as a white man, aged between 55 and 65 years, with a large stomach and dark chin-length hair with blond or lighter ends. He was wearing beige trousers, a checked shirt and a yellow patterned baseball cap and is believed to have an Irish accent.
The incident occurred on 1st July 2014.