The rapid-acting, deadly chemical was found on drinking glasses and a teapot in the room at the luxury Grand Hyatt Erawan hotel, and interviews with relatives of the dead revealed there had been a dispute over debt related to an investment, police said.
The six were all of Vietnamese ethnicity, two of those US nationals, and were found dead late on Tuesday.
The US Federal Bureau of Investigation had assisted police with the investigation, police said.
"We found cyanide in the teacups, all six cups we found cyanide," Trirong Phiwpan, commander of the Thai police evidence office, told a news conference on Wednesday.
Thai Prime Minister Srettha Thavisin visited the hotel, later saying the incident was not random. (AP PHOTO)
"After staff brought teacups and two hot-water bottles, milk and tea pots ... one of the six introduced cyanide."
The results of an autopsy were expected within the next day, police said.
Vietnam's government said its embassy in Bangkok was closely co-ordinating with Thai authorities on the case, while the US State Department said it was monitoring the situation and local authorities were responsible for the investigation.
The Grand Hyatt Erawan, operated by Erawan Group , has more than 350 rooms and is in a popular tourist district known for luxury shopping and restaurants.
News of the deaths, initially reported by some Thai media as a shooting, could be a setback for Thailand as it bets heavily on its vital tourism sector reviving an economy that has struggled since the pandemic.
Thailand is expecting 35 million foreign arrivals in 2024, up from 28 million in 2023 who spent 1.2 trillion baht ($A50 billion).
Prime Minister Srettha Thavisin on Tuesday urged a swift probe into the issue to limit the impact on Thailand's travel sector.
