Christian Bart, 54, registered a complaint with Muang Pattaya Police at 3am.
He said he was riding a motorcycle and enjoying the city by night and stopped near the hotel in north Pattaya.
The woman approached him abruptly and gave him a hug, but grabbed his necklace before he was aware what was happening and fled.
Police were checking CCTV cameras in the area for recordings of the incident.