The singer, real name Rory Graham, whose hits include Human and Giant, played the iconic, award-winning independent venue as part of a national campaign to help revive grassroots music venues.
He lives locally, near Heathfield, and had played The Forum, a former public toilet on the Common, before he became a big star, although on that occasion it was not a full house.
Monday’s gig was sold out in minutes when tickets went on sale last month, and the star was rewarded with a trophy from the music venue.
The Forum tweeted: “Brit awards, Shmit awards. All things like that are just put to shame when you win the ‘Sold Out show at The Forum’ trophy.
“Well done to @RagNBoneMan for finally reaching his peak!
“Ladies, gents and everything in between… Rag N Bone Man is a sell-out.”