The blaze broke out late yesterday in the "narrow and winding" warren of small private rooms, complicating the search for victims, the fire brigade said.
Several doors had to be rammed open to check whether anyone had fallen unconscious as the thick smoke and fire spread through the club, which occupies the ground floor and cellar of a high-rise building and neighbours a care home for the elderly.
"It was a lot of work to break into each room and to check for victims," a fire brigade spokesman said in a statement, adding that 80 rescue workers had been dispatched to the scene.
The Steam Works club in the central Schoeneberg district is made up of saunas and common areas, as well as 60 small rooms, each with a bed inside, and is a well-known meeting place for Berlin's gay scene.
When firefighters arrived, about 25 club guests who had fled the blaze were standing on the street, some dressed only in towels, and directed emergency service workers to where victims might be.
The cause of the blaze was not immediately clear.