Foster was held on $75,000 bail at the Santa Clara County jail after being booked on charges related to domestic violence and possessing an assault rifle in Los Gatos, Calif.
"The San Francisco 49ers' organization is aware of the report regarding Reuben Foster," the 49ers said in a statement. "We take matters of this nature seriously and are gathering all pertinent information."
The 23-year-old Foster was also arrested Jan. 12 in Tuscaloosa, Ala., on a misdemeanor second-degree marijuana possession charge.
A year ago, Foster was sent home from the NFL Combine in Indianapolis after he had a run-in with a hospital employee. He failed a drug test at the Combine as a result of providing a diluted sample.
On the field in 2017, Foster ranked second on the team in tackles with 72 despite missing five games because of a high ankle sprain and another game with back spasms. He was named the NFL's Defensive Rookie of the Month in November.
Foster capped his college career at Alabama in 2016 by winning the Butkus Award that goes to the nation's top linebacker. He slid to the bottom of the first round as the No. 31 pick in the 2017 NFL Draft because of the character and medical issues (shoulder surgery after his final season with the Crimson Tide).
The NFL typically investigates any accusation of domestic violence against a player. Those who are found guilty for the first time could face up to a six-game suspension from the league.
49ers cornerback Tramaine Brock was charged in April with felony domestic violence after he was accused of attempting to strangle his girlfriend. He was subsequently cut by the team.