The man – described as being in his 30s, white, bearded and with short dark hair – was said to have been “acting in a lewd manner” on La Coupe Beach at around 14:00.
Police reported that he drove to the area in St. Martin in a blue Mitsubishi van.
His alleged “indecent” behaviour was then said to have occurred at the far end of the beach away from the steps.
Anyone with any information is asked to contact Police on 612612 or anonymously via Crimestoppers on 0800 555 111.