The incident occurred on Sunday, July 18 at around 3.30pm near the Oceanarium. According to Dorset police, who are appealing for witnesses, the victim was with her friends playing with a ball, when it landed in front of the teenage boy. The boy returned the ball to the group, and then proceeded to chat to the victim.
He then reportedly pulled the 15-year-old deeper out into the sea, where he is alleged to have raped her.
The offender told the victim that he was 17 years old and was travelling to Birmingham.
The suspect is described as having tanned skin and being possibly of Pakistani descent.
He is said to be between 5ft (152cm) and 5ft 7in (170cm) tall and to have a thin-but-muscular build.
The boy had short dark hair which was pushed back and wore black or grey swimming trunks.
Police are asking anyone who might have witnessed the event to come forward.
In particular, they are asking people to check their photographs and any videos they might have taken to see if they have captured anything of interest.
“I am particularly keen to speak to anyone who recognises the boy from the description given or may have seen him on the beach on Sunday, July 18.
“This incident will understandably cause concern for the wider community and we would like to remind the public that officers from the local neighbourhood policing team will be carrying out patrols in the area and can be approached with any concerns.”
Anyone with information is asked to contact Dorset Police at www.dorset.police.uk, via email [email protected] or by calling 101, quoting occurrence number 55210115587. Alternatively, to stay 100 per cent anonymous, contact the independent charity Crimestoppers online at Crimestoppers-uk.org or call Freephone 0800 555 111.