Justin Bieber: San Francisco Demands Answers ... Who Graffitied Our Town?

San Francisco is on the hunt for the person who graffitied a sidewalk to promote Justin Bieber's new album ... and they want his record label to snitch out the culprit.

City Attorney Dennis Herrera fired off a letter to Universal Music Group ... claiming the Bieber graffiti won't come off ... even after a few recent rain storms.

Herrera said the Public Works crews have been called in to remove it, but that doesn't come cheap.

It's unclear if the city prosecutor wants the graffiti artist to foot the bill, or whether he wants to prosecute the culprit for malicious mischief.

Bieber's album, "Purpose," debuted at #1 on the Billboard charts ... so mission accomplished.