Katt Williams -- Sued for $5 Million for Allegedly Punching Assistant

More legal trouble for Katt Williams -- the comic has just been sued for $5 million after allegedly socking his female assistant .... and causing her permanent injuries.

Melissa Ishage filed the lawsuit in L.A. County Superior Court, claiming she'd been working as Katt's personal assistant -- but on October 6th, things suddenly went sour at Katt's house ... when Katt attacked her, punching her and causing her "serious and permanent injuries."

Melissa doesn't explain why Katt flew into a rage -- but claims she had to go to the hospital as a result of the beatdown ... and now she wants at least $5 million from Katt to call it even.

Melissa's attorney tells us, his client claims Katt struck her in the head and knocked her to the ground. The lawyer says police were called to the scene but no one was arrested. The incident is currently under investigation.