Two girls aged ten and 11 say they were sexually assaulted in the sea off Bournemouth beach yesterday.
Officers were called at 3.45pm to reports a man had touched the girls inappropriately while they were swimming near the pier.
Dorset Police said they arrested a 43-year-old man from London on suspicion of sexual assault.
The girls' allegations are the latest reports of sexual assault in the seaside town.
Earlier this month, police said they were looking for a group of men after a 17-year-old boy claimed he was sexually assaulted in the sea.
Meanwhile, a 19-year-old man is awaiting trial after a 15-year-old claimed he raped her in the sea in front of thousands of holidaymakers.
A spokesman for Dorset Police said today: 'Detectives are appealing for witnesses or anyone with information to come forward as they investigate two reported sexual assaults at Bournemouth Beach.
'We received a report at 3.45pm on Sunday of a disturbance in the area of Pier Approach. Officers attended and it was reported that two girls - aged ten and 11 years - had been touched inappropriately by a man when they were in the sea near to the pier.
'Following inquiries, a 43-year-old London man was arrested on suspicion of sexual assault.'
Detective Constable Cameron Carter, of Bournemouth CID, said: 'We are carrying out a detailed investigation into these incidents and I am keen to hear from anyone who witnessed what happened who has not already spoken to police.