Per Adam Taylor of the California Highway Patrol, Westbrooks was charged with "suspicion of receiving stolen property, carrying a loaded firearm, carrying a stolen loaded firearm, grand theft of a firearm and possession of a high-capacity magazine" after being stopped for speeding.
He was released from Kern County Jail after posting a bond of $55,000.
Westbrooks is in his fourth year with the team and has appeared in all three games for the Rams this season, totaling five tackles and a sack. He started in Week 1 in place of Aaron Donald while the Pro Bowler was returning from his holdout.
The 26-year-old's arrest happened the day after the Rams' 41-39 win over the San Francisco 49ers Thursday.
Los Angeles is scheduled to face the Dallas Cowboys Sunday and the Rams have not yet commented about Westbrooks' status.
The defensive lineman had also been arrested in March on suspicion of domestic violence, but he was not charged.