The boy was playing in Halton Community Centre park when he was approached by an offender who asked the youngster to follow him to a wooded area.
The boy went with the suspect and was then sexually assaulted.
The victim reported the incident to his family who called the police.
The incident happened at about 5.30pm yesterday (Tuesday, September 7th)
The offender is described as white, possibly a teenager, wearing a white polo shirt with buttons down the front, black trousers and black trainers.
A manhunt is under-way and a team of detectives are carrying out a number of enquiries to try and identify the offender.
The boy and his family are being supported by officers.
Detective Chief Inspector Steve Ryder, of West CID, said: “This was an extremely serious offence and we are doing all we can to identify the offender. While we do have a number of lines of enquiry, I would appeal to anyone who saw anything suspicious in the park yesterday afternoon or anyone matching the description of the suspect to get in touch. It was obviously a fine and sunny day yesterday so I’m sure the park was busy with people at this time.
“I would also urge people to be extra vigilant if they are out and about and to make sure they know where their children are at all times and that they are suitably supervised.”
Extra police patrols, including mounted officers, will be in the area over the next few days and anyone with information or concerns should feel free to approach them.
Anyone with information should contact us on 101 quoting log 1103 of September 7.
Alternatively contact independent charity Crimestoppers on 0800 555 111.