A sin in Sin City.

Tarek El Moussa was cited for battery after he allegedly got into a physical fight with another man while in Las Vegas on Thursday, according to TMZ.

The fight involving the HGTV star, 43, reportedly occurred at the Palazzo at the Venetian Resort.

The Las Vegas Metropolitan Police Department confirmed to The Post that officers responded to a battery incident in the 3300 block of South Las Vegas Boulevard. Two males were involved in a physical altercation resulting in one being cited.

Witnesses claimed that El Moussa allegedly kneed the other man in the head and knocked him out. One witness described the incident as “bad” and said the man looked seriously hurt.

The alleged victim reportedly refused medical attention.

A source close to El Moussa told TMZ he was allegedly defending his “elderly father” after the man allegedly got out of line with the star’s dad.

Police arrived on the scene and cited El Moussa for battery but didn’t arrest him, TMZ reported.

The Post has reached out to Tarek for comment.

El Moussa was in Las Vegas with his wife, Heather Rae Young, 37, and their parents for his speaking event.

Heather shared footage from the getaway on Friday, June 6, writing, “Not our average Vegas trip.”

“Watched Tarek do his thing on stage (so proud!) and made it extra special by bringing our parents along for the ride,” Heather continued. “We got to show them a glimpse behind the scenes — our world, our work, and of course, the glam.”

“Grateful for these moments that we get to spend together,” the “Selling Sunset” alum added.

The couple tied the knot in October 2021. They share son Tristan, 2.

El Moussa is also dad to daughter Taylor, 14, and son Brayden, 9, who he shares with ex-wife Christina Haack. The former couple split in 2016 after seven years of marriage.

Haack, El Moussa and Young all work together on the HGTV show “The Flip Off.” During filming, Haack split from her husband Josh Hall.