The man, who was later arrested, broke away from the line outside the bookstore, lunging at the unsuspecting London-born R&B singer and "punched her in the side of the head," her spokesman told the BBC.
The three-time Grammy Award nominee was promoting her autobiography "Dreams" at the Waterstones bookstore when she signed the alleged attacker's book and "as she looked up, he just punched her," one witness told British news agency.
"She was running out with her hand over her eye, and I just saw a man on the floor," another witness told the BBC.
"Suddenly, the security all jumped on him and they were trying to pull him out and he was just laughing. He thought it was funny."
The "Bleeding Love" singer sought medical attention but was not seriously hurt. Still, she was "understandably badly shaken," said her spokesman.