Fifteen people were also seriously injured, according to police in Pattaya, a popular beach resort south of Bangkok renowned for its racy nightlife.
"So far there are six dead from the boat," Pattaya police chief Colonel Suwarn Chiewnawintawat said.
Three Thais, a Chinese and two other - as yet unidentified - foreigners were among the dead.
Television footage showed stunned tourists being led to safety on shore where they were met by dozens of ambulances along Pattaya's neon-lit beachfront.
Playing down earlier reports that people remained trapped in the stricken vessel, Colonel Chiewnawintawat said all of the other 150 passengers had been plucked from the sea.
The double-decker ferry sank on Sunday afternoon near Koh Larn, a small island close to Pattaya which is popular with daytrippers.
Police said they are probing the cause of the accident, but local media reports say engine problems on the overcrowded ferry forced passengers to one side of the boat, which caused it to list and take on water.
Consular officials from the Australian Embassy in Bangkok say they're working with local authorities to find out if any Australians were on board.
Russian news reports said three Russians, including a child, had been taken to hospital in a serious condition.
"At least half of the passengers were Russian tourists," a Russian embassy official in Bangkok was quoted as saying by the Ria Novosti news agency.
Map: Pattaya is one of Thailand's most popular resort areas
Renowned for its strip clubs and bars and cheap accommodation, Pattaya is one of Thailand's most popular resort areas and has become wildly popular as a package holiday destination for Russians.
Thailand drew 22 million tourists last year, but is struggling to shake off a reputation for lax safety standards after a series of incidents - many of them fatal.
In May, more than 100 people were rescued from a tourist ferry which began to sink in rough seas near the tourist island of Phuket after it was hit by a big wave.
Four people, including tourists died in a nightclub fire in August 2012 also on Phuket, while there have also been slew of high-profile cases of foreigners being murdered, drugged or caught up in tourist cons.
In July, an American tourist was allegedly stabbed to death by three Thai musicians after he refused to stop singing at a bar. His death came weeks the murder of another US citizen in a row with a taxi driver in Bangkok.
Diplomats from China and the European Union have voiced concern at the number of fatal incidents involving their tourists.
In recent years the kingdom's tourist-friendly image as "the Land of Smiles" has also been tarnished by political violence and devastating floods.