The Los Gatos-Monte Sereno police confirmed Sunday night that Foster was arrested earlier in the day on charges related to domestic violence, threats and possession of an assault weapon.
Police responded to a disturbance call placed at approximately 9:15 a.m. Sunday at a Los Gatos residence. Upon arrival, officers spoke to Foster and the alleged victim. The result of the initial investigation was to arrest Foster, 23.
Foster was then transported and booked at the Santa Clara County Main Jail, where he was held on $75,000 bail. He was released on bail Sunday night. The Los Gatos-Monte Sereno Police said the incident remains under investigation.
The San Francisco Chronicle first reported Foster's arrest, and the San Jose Mercury News reported the additional charge related to suspicion of possession of an assault rifle.
The 49ers released a statement Sunday night indicating that they were in the process of gathering facts about the situation.
"The San Francisco 49ers organization is aware of the report regarding Reuben Foster," the statement said. "We take matters of this nature seriously and are gathering all pertinent information."
For Foster, Sunday's arrest comes on the heels of a Jan. 12 incident in Tuscaloosa, Alabama, where he was charged with second-degree marijuana possession, a Class A misdemeanor in Alabama. He was released that night on $2,500 bond.
Foster's arrests could lead to discipline from the NFL, up to and including a suspension. Foster's cases both stand to be reviewed under the league's policies on substance abuse and personal conduct.
Both arrests follow a successful rookie season for Foster, in which he was second on the Niners in tackles, with 72 in 10 games. The arrests are added to a list of off-the-field incidents that contributed to Foster's falling to San Francisco with the No. 31 pick in the 2017 NFL draft.
Foster initially raised some red flags at last year's NFL scouting combine, where he was sent home early after an argument with a hospital worker. He also had a urine sample taken there show up as diluted, which, according to the NFL's policy on substances of abuse, was treated as a positive test.
Because of the diluted sample, Foster entered the league as part of the substance abuse program.
Despite the incidents in Indianapolis, 49ers general manager John Lynch said after drafting Foster that he and the organization were comfortable with Foster's character and that Foster would be able to avoid trouble off the field. After the January marijuana arrest, Lynch told SBNation.com at the Senior Bowl that Foster had to figure out how to avoid off-the-field issues.
"He's got to stay -- he knows it -- he's got to stay clean," Lynch said. "We all know how special a player he can be when he's right. So he's got to figure out how to stay healthy and stay out of trouble, but we believe he will do that."
Foster's arrest is the second related to domestic violence since Lynch and coach Kyle Shanahan joined the 49ers last year.
In April 2017, cornerback Tramaine Brock was arrested in Santa Clara on a felony domestic violence charge. The 49ers released him the next day. The charges against Brock, who is now with the Minnesota Vikings, were dropped in August, and the NFL did not pursue any further discipline.
At the time of Brock's release, Lynch said the Niners' handling of such situations would be handled on a case-by-case basis.
"I think we have been consistent, we have said that character is important to us, football character," Lynch said then. "Football character means a lot of different things. As Kyle articulated very well, and I feel the same way because we kind of forged these philosophies together, as those situations arise -- and hopefully there won't be a lot of them -- we're going to treat each one of them as a unique and different situation."