Officers say the attack happened along the stretch of beach along Les Bas Courtils, opposite the junction with Les Grandes Maisons Road, on Saturday 5 June.
The owner of the dog is described by police as a white woman in her late 30’s/early 40’s.
She is of medium height, had brown hair and “spoke with a French accent”. At the time of the incident, she was wearing a pink jumper.
The dog involved is believed by police to be a Border Collie.
Anyone with information regarding this incident is asked to contact PC 101 Wilen, PC 18 Winslade or PC 106 Thomas on 222 222 or to contact CrimeStoppers anonymously on 0800 555 111.