Alert beach boys at Nai Harn, in southern Phuket, spotted Niruj Indharat stealing the bag from a tourist whose name was given as Andre.
The 21-year-old thief, from Had Yai, had pretended to be a tourist before taking the bag.
Later, at Chalong Police station, Khun Niruj was charged. In the stolen bag were a Samsung mobile telephone, 1300 baht in cash and Andre's passport.
Nai Harn beach vendor Song Saengtong said it wasn't the first time that beach boys had had to defend their reputations.
''The beach boys are often accused when valuables disappear at the beach,'' he said. ''But when you think about it, their livelihoods depend on their honesty.
''The problem is that because Phuket's beaches are so crowded these days, it has become more difficult to keep a watch on everyone.""