The Riverside County Sheriff's Office said deputies assigned to Trump's rally in Coachella Valley contacted the driver of a black SUV at a checkpoint just before 5 p.m. local time.
The driver, a 49-year-old man, was found to be illegally in possession of a shotgun, a loaded handgun, and a high-capacity magazine, the sheriff's office said.
That driver, identified as Vem Miller of Las Vegas, was taken into custody.
Miller was later booked at the John J. Benoit Detention Center for possession of a loaded firearm and possession of a high-capacity magazine. He was released Saturday on $5,000 bail.
The sheriff's office said the incident did not impact the safety of Trump or anyone at his event.
Media members, as well as VIP ticket holders, were routed through a number of intersections manned by state and local law enforcement officers before arriving at a large, grassy area where drivers were asked to open hoods and trunks, and each vehicle was searched by a K-9 officer. Other general ticket holders were directed to a site roughly 3 miles away from the rally, where they were boarded onto buses and driven to the site.
Trump's campaign did not immediately respond to a request for comment on the arrest.
Riverside County Sheriff Chad Bianco planned an afternoon news conference to discuss the arrest.
Police asked anyone with information to call the Palm Desert Sheriff's Station at 760-836-1600.
Further information was not immediately available.
The Associated Press contributed to this report.
This is a developing story. Check back for updates.