Over 50 firefighters were involved in tacklin the blaze at Envy nightclub on Thursday evening.
Detective Sergeant Gavin McLaughlin said: “Police became aware of a fire at Envy nightclub just before 7pm on Thursday evening.
“Officers attended immediately and evacuated nearby properties and closed a number of roads as our colleagues in NIFRS dealt with the fire.
“The NIFRS has ruled this to be deliberate ignition, and so we are treating this as arson.
“The investigation is still at an early stage and we have no motive for the attack and we are keeping an open mind.
“I want to appeal to anyone who saw a male acting suspiciously in the Strand Road area last night between 6.30 pm and 6.45 pm, or anyone who may have witnessed any other suspicious activity to get in touch with detectives in Strand Road on 101, and quote reference number 1523 of 09/01/20 .”
Alternatively, information can also be provided to the independent charity Crimestoppers on 0800 555 111 which is 100% anonymous and gives people the power to speak up and stop crime.