An 11-year-old boy suffered a serious indecent assault at the harbour on Wednesday evening (October 22) at about 6.30pm.
The suspect is described as white, about 30, with blond hair. He was wearing a black t-shirt and grey jogging bottoms.
The attacked happened at the north-eastern side of the Harbour Peninsula.
Detective Sergeant Bonnie Cowley, of Eastbourne CID, said, “Whilst serious, this is an isolated incident and enquiries are continuing.
“Neighbourhood policing teams are carrying out high-visibility patrols in the area to offer reassurance to visitors and residents.”
Anyone who noticed anything suspicious in the area or who may have other information that could help officers is asked to email email@example.com or phone 101, quoting serial 1605 of 23/10.
Alternatively they can contact the independent charity Crimestoppers free and anonymously on 0800 555111.