TMZ Sports has learned ... Boykin was indicted for 2nd degree felony aggravated assault-serious bodily injury by a grand jury in Tarrant County, TX on Friday.

At the time of the incident, Boykin's longtime GF, Shabrika Bailey, claims he choked her and broke her jaw during a physical fight before finally taking her to the hospital for treatment.

As for Boykin, he's vehemently denied any physical altercation took place.

Boykin was a stud at TCU but had issues off the field -- including a fist fight with cops days before the Alamo Bowl.

He was undrafted -- but later signed a contract with the Seattle Seahawks and seemed to be doing well until the domestic violence arrest.