A librarian at Palatine Library in St Annes Road, South Shore, was attacked after asking a man to leave, police said.
The worker, a 54-year-old man, had his hand twisted and was shoved in the chest by the assailant.
A CCTV image of a man they want to speak to was released this afternoon.
The attack was reported at 3.30pm on Monday, August 20 and happened earlier that day.
"An unknown offender went inside the library and became embroiled in an argument with a member of the public," a police spokesman said.
"The offender was asked to leave and has assaulted a staff member."
Information can be reported by calling 101, quoting WA1820050.