He allegedly approached a group of teenagers at the Hedgerow Walk play park and assaulted a 15-year-old girl.
It happened at between 6.30pm and 7pm a man on Wednesday July 12.
Her friends went to find help and found a couple who intervened.
The police have appealed to that couple to come forward.
A police spokesman said: “We would like to speak this couple, or anyone else who was in the area at the time as they may have vital information.”
The 15-year-old girl was not injured.
A 40-year-old man from Southampton has been arrested on suspicion of sexual assault. He has been released on police bail until October 16.