About 750 people were celebrating at Neighbours in the 1500 block of Broadway when someone poured gas on the carpeted stairway leading to the second level of the bar, and ignited it, Seattle police said.
Police are investigating the fire as an arson.
Two patrons at an upper level bar used a fire extinguisher to put out the flames. As staff was trying to clear the nightclub, someone pulled a fire alarm that activated the indoor sprinkler system.
Police on Monday released several still images taken from surveillance video shot during the party. Detectives have not said the man in the photos is a suspect in the case, but they do want to interview him.
Anyone who recognizes the man is asked to call Arson/Bomb Squad detectives at (206) 684-8980. Police are also asking for everyone who was at the party to look over their own photos or videos to help them identify the mystery man.