TMZ reports:
The former NFL star wide receiver was busted in Calabasas, CA early Monday morning.
According to law enforcement ... Keyshawn was involved in a fight with his ex-girlfriend when things allegedly got physical. Sources say he smashed her phone, causing a gash to her hand.
Our sources say the two had an up-and-down relationship and were arguing over marriage.
Johnson was formally arrested for misdemeanor domestic battery and booked.
Bail was set at 20K. Keyshawn made bail and is now out.