The accident took place on the Chao Phraya river in Ayutthaya, a UNESCO World Heritage site located some 80 kms (43 miles) north of the Thai capital Bangkok.
At least 37 were injured and many are still missing as rescue workers continue to search for survivors into the night.
The dead include seven women, four men and one boy, Rewat Prasong, deputy governor of Ayutthaya province, told Reuters.
The passengers were on their way back from a religious activity when the double-decker boat hit the bank of the river before sinking, district chief Suchon Phaitirat told Reuters.
The accident on September 18 comes as Thailand expects to welcome some 33 million visitors this year, a record. Road and boat accidents involving tourists are common in Thailand where safety standards are sometimes well below international norms.