The blaze occurred at the Mountain B pub in the Sattahip district, 100 miles southeast of the capital, Bangkok about 12.45am (6.45amBST) on Friday.
Manop Theprith, from a private emergency service group, said 40 people were injured in the blaze which took two hours to extinguish.
One witness, named Nana, told PPTV: “I’m quite shocked but I am lucky that when I saw the fire, I could get myself together and get out of there.”
According to a DJ at the club, who was not named , the fire spread in just a minute before customers were ushered out of the venue.
Nana added that she saw several security guards with their clothing on fire. She also heard multiple explosions before seeing the ceiling ablaze.
She said: “The fire started at the top right corner of the stage.
"The singer must have seen it too, so he shouted 'fire' and threw away the microphone."
A waitress at the venue, Thanyapat Sornsuwanhiran, told Thai television reporters that she saw smoke near the stage.
"I shouted 'fire' to customers, and I was near the doors, so I directed them out. I kept shouting 'fire, fire' and the security guards were also helping lead people out.”
Local TV footage showed people fleeing the fire and emergency workers putting out flames and looking through the burnt-out premises, with shoes and bottles strewn across the ground.
The cause of the fire is not yet known.