After releasing his debut solo album on Friday the former One Direction star announced the intimate show, at a north London club, via Twitter.
The Garage in Highbury has a capacity of just 600 people so it was first come, first served.
Tickets were £10 and Newsbeat caught up with a few of the lucky fans who managed to bag one.
Chiara, 21, didn't even manage to get dressed: "I'm wearing my pyjamas with the Little Mermaid and everyone inside was saying, "are you wearing this tonight at the gig" and I actually might as it's fancy."