Aaron Hernandez -- CHARGED WITH ASSAULT ... Over Jailhouse Fight

Aaron Hernandez has been charged with assault and battery for a ferocious attack on a defenseless inmate last month ... TMZ Sports has learned.

According to Sheriff Thomas Hodgson ... the charge is a misdemeanor offense.

TMZ Sports broke the story ... Hernandez attacked an inmate who was wearing handcuffs on February 25th. Our sources tell us the victim had been talking smack to Hernandez.

Hernandez was supposed to be isolated for his own protection.