Tarek El Moussa got into a physical altercation in Vegas — and while he walked away with a citation, sources close to the HGTV star say he was stepping in to defend a family member.

A spokesperson for the Las Vegas Metropolitan Police Department tells TMZ … the “Flip or Flop” star and another male were involved in an altercation last Thursday at The Palazzo at the Venetian Resort.

Witnesses who were at the scene tell TMZ … Tarek delivered a brutal knee to the man’s head, knocking him out cold. One witness called the incident “bad,” saying the other guy looked seriously roughed up.

It’s currently unclear what started the fight, but we’re told it escalated quickly. Cops took a crime report and cited Tarek for battery, but did not arrest him. We’re told the alleged victim refused medical attention.

A source close to Tarek tells TMZ … he was stepping in to defend his elderly father, with the source suggesting the confrontation was sparked by the man getting out of line with his dad.

Tarek’s wife, Heather Rae El Moussa, was with him at the time — but we’re told she had no role in the scuffle — sharing some pics and clips from the family’s Vegas trip on her Instagram.