The girl was reported to be one of two under-age children being offered for prostitution at the Music 999 karaoke bar in Kwang Road, Vichit, south of Phuket City.
Acting on a tip-off, a plainclothes policeman went to the karaoke and paid 3000 baht to take the 14-year-old to a hotel for the night.
Although the informant told police that two minors were involved, officers from Vichit Police Station only found one girl when they raided the bar about 1am today.
About 10 people at the karaoke were questioned and one tested positive for drugs.
The mother of the 14-year-old was arrested. She told police that she sold her daughter into prostitution to cover the rent on their apartment.
The girl will be taken into care. Police are expected to follow leads to try to trace the second girl.