The child was enjoying playing in the sand dunes on Studland beach when the man, said to be aged between 50 and 70, struck.
He was described as having a hare lip, brown hair, and wearing a green jacket. He carried a back pack and wore sunglasses.
The attack was reported to Dorset police at 6pm on Friday. Officers have appealed for anyone with information to contact Bournemouth CID.
In a statement, a police spokesperson said: "The girl was playing in the sand dunes when she was approached by the suspect, who sexually assaulted her."
Det Sgt Neil Wright said: "This is a very unusual incident in a particularly safe area of the country."
Witnesses should call 101 and ask for Det Sgt Wright, quoting incident number 551/10.