Somerville, 52, the former Bronski Beat frontman and original singer of the song, happened to be walking his dog in the area.
The Scot immediately joined the surprised busker in singing the song which got to number three in the charts.
More than 300,000 people have watched a video of the impromptu version after it was uploaded onto YouTube.
A member of Onkel Bernis Welt, who filmed the footage, wrote: “What a huge surprise for this street musician and us.
“We were filming him singing the song Smalltown Boy by Bronski Beat, when suddenly Jimmy Somerville (the original singer) comes up and joins him singing.
“Awesome – things like that only happen in Berlin.”Post published in: World News