A member of the public reported hearing a bang in the Graham Gardens area at 6am on Saturday.
It’s thought that petrol was poured through the shutters of the office, but did not ignite properly.
Fire damage was found behind the shutters, and a forensic examination is under way.
At this stage it is not thought that an explosive device was used.
The party’s Lagan Valley MLA, Trevor Lunn, condemned the attack: “This is an attack on democracy. There is no place in our society for this violence that should be condemned by all.
“This attack will not deter us from continuing to work on behalf of the people of Lagan Valley.
“This is the latest in a number of attacks on Alliance Party offices, but we will not give in to violence and intimidation. Alliance will continue to work to achieve a shared future for everyone.
“I would urge anybody with any information about this attack to contact the police.”