The controversial hand gesture went down during the Dustin Poirier and Conor McGregor in Las Vegas over the weekend.
In a video making the rounds, the 65-year-old Braveheart actor is seen standing in the crowd and saluting Trump.
The ex-president waves to onlookers and walks past almost in an instant.
The clip is quick, but Gibson stands out because he's the only one frozen in his stance.
Twitter user @petsh0pboy posted the video, laughing at the actor and saying, "Lmao mel gibson saluted trump at ufc and got completely ignored."
It quickly gained attention with several divided responses, much like Trump's appearance at the T-Mobile Arena.
When the former president showed up, he collected a mix of cheers and boos. Almost immediately, it was 2020 all over again.
Gibson, who's had his fair share of backlash for anti-Semitic comments he's made, had some supporters with many labeling him a "patriot."
"A true Patriot! Not afraid like the other Hollwood [sic] spineless!" one person responded. "Mel Gibson, always the Patriot," stated another.
"Mel Gibson is a Patriot who literally played The Patriot. Wokesters hate him because the only thing they salute is their tall latte they stapled a dress on and put their d--- in," shared a third.
Others weren't impressed.
"Two whole Assholes," someone replied. "The Mel Gibson news is the least shocking thing ever, but also... who cares? One loser saluting another. Big whoop," added another.
Gibson has never been one to talk about his political views. According to Film News, it's because the Academy Award winner doesn't want to ruin the "sense of anonymity."
The actor has said plenty in the past to cast a shadow on his career.
Many video watchers aren't surprised about Gibson showing support for Trump.
"Why are people surprised Mel Gibson is a Trump supporter. They are literally cut from the same cloth. They both as a person are awful. Both do great work in their respectful fields," a Twitter user wrote. "'Mel Gibson "cancelled"? The guy is a cockroach, has spewed the worst kind of anti Semitic hate speech and racist bullshit it just doesn't stick. But apparently this is the breaking point for some people? Like now? Saluting some dude is like number 5000 on reasons to hate him," added another.