The contractors were working on a fire safety system at the headquarters of the Siam Commercial bank on Sunday night, the bank said in a statement.
They apparently suffocated after a gas mixture which depletes oxygen was released.
Another seven were injured in the incident, which may have been caused by contractors' "negligence", it added.
A bank spokesman told the BBC the incident took place in a vault storing documents in the head office in Bangkok.
The bank said at around 21:30 local time on Sunday (14:30 GMT), an aerosol system called pyrogen was "inadvertently activated and exhausted all the oxygen in the area" as contractors were working on it.
Pyrogen involves a mixture of gases including potassium carbonates, nitrogen, carbon dioxide, carbon monoxide, nitrogen oxide and ammonia.
The system works by removing all oxygen from the air to stifle a fire.
The Bangkok Post reported that firemen encountered delays as they tried to reach the victims due to tightly locked doors.
The bank, one of Thailand's largest financial institutions, added that it would provide assistance to those in the accident and operations would continue normally.
Correspondents say that fatal industrial accidents remain rare in Thailand.
A toy factory fire in 1993 which killed nearly 200 people and injured another 500 is considered the worst the country has seen.
Eight people have died in a chemical accident in the basement of a Bangkok bank, officials have confirmed.