The fire occurred at the Mountain B pub in the Sattahip district, 100 miles southeast of the capital, Bangkok, at around 12:45 a.m. (6:45 a.m. BST) on Friday.
Manop Theprith, from a private emergency services group, said 40 people were injured in the blaze which took two hours to burn out.
A witness, named Nana, told PPTV, “I’m quite shocked but I’m lucky that when I saw the fire I was able to pull myself together and get out of there.”
According to an unnamed DJ at the club, the fire spread within just a minute before patrons were forced out of the venue.
Nana added that she saw several security guards with their clothes on fire. She also heard multiple explosions before seeing the ceiling go up in flames.
She said: “The fire started in the upper right corner of the stage.
“The singer must have seen it too, so he yelled ‘fire’ and threw the mic away.”
A waitress on site, Thanyapat Sornsuwanhiran, told Thai TV reporters she saw smoke near the stage.
“I yelled ‘fire’ to customers, and I was near the doors, so I got them out. I kept yelling ‘fire, fire’ and the security guards were also helping to make get people out.”
Local television footage showed people fleeing the blaze and rescuers extinguishing the flames and looking through the burned premises, with shoes and bottles strewn on the ground.
The cause of the fire is not yet known.