The NFL running back was blindsided with an assault and battery lawsuit at an autograph signing over the weekend -- filed by the valet he allegedly attacked in April ... and TMZ Sports has the video.

Here's what went down ... CJ was signing pics and memorabilia for fans at a minor league baseball game in FL on Saturday ... when a process server patiently waited in line for her time to meet the All-Pro back.

Once the server got her chance ... she placed the papers right on the table for Johnson, who nearly puts his John Hancock on the complaint and summons until he realizes what's in front of him.

In the civil suit, the alleged victim states he was seriously injured, lost work 'cause of the incident, and has medical bills to pay. It's unclear exactly how much he's asking for ... but it's at least $15K.

It's a clever tactic, but not the first time we've seen this move used -- Warren Sapp was duped at a meet-and-greet back in February.