A consensus All-American in high school, Foster was regarded as the best linebacker prospect of his class before committing to Alabama, where he played from 2013 to 2016.

During his professional career, Foster has been involved with a number of legal issues and other controversies, including multiple arrests, which led to his release from the 49ers in just his second season.

During his freshman year in a game against Columbus (GA) Carver, Foster had a heralded performance against blue chip running back Isaiah Crowell.

[5] Foster eventually moved back to Alabama and enrolled at Auburn High School in April 2012,[6] where he became teammates with Rashaan Evans.

[7] In his senior year at Auburn High, Foster recorded 80 tackles (70 solo) and two sacks, helping the Tigers advance to the quarterfinals of the Class 6A playoffs.

[8] Foster participated in the 2013 Under Armour All-America Game, finishing with six tackles and 2.5 for loss, earning Defensive MVP honors.

[9] Everything you've heard about Reuben Foster, it's all true – he's big, fast, strong, explosive and finishes off his tackles as good as any player I've ever seen in this business.

In July 2011, Foster verbally committed to Alabama,[16] citing the NFL pedigree of head coach Nick Saban and defensive coordinator Kirby Smart as the decisive factor.

Competing to fill the vacancy at inside linebacker left by Butkus Award winner C. J. Mosley, Foster was injured during spring practice on April 5, 2014.

[citation needed] In his junior year, Foster took over as starting inside linebacker from Trey DePriest, and finished second on the team with 73 total tackles, eight for a loss (−24 yards), on the season.

[26] On January 18, 2017, it was reported that Foster and Alabama teammate Jonathan Allen declined their invitations to play in the 2017 Senior Bowl.

On March 8, 2017, Foster attended Alabama's pro day and set scheduled private meetings with NFL teams to discuss the incident that took place at the combine.

[42] He joined a revamped linebacker corp that consisted of NaVorro Bowman, Ahmad Brooks, Elvis Dumervil, and Malcolm Smith.

[43] He made his professional regular season debut and first career start in the San Francisco 49ers' season-opener against the Carolina Panthers and recorded three solo tackles and deflected a pass during their 23-3 loss.

Foster finished the 49ers' 40-10 loss to the Dallas Cowboys with five combined tackles, but left in the third quarter after suffering a rib injury.

[53] Regarding the decision to claim him, Redskins senior vice president of player personnel, Doug Williams, stated that he and the rest of the team were fully aware of the allegations that led to his arrest, adding that Foster would have to go through the legal process and accept any other punishment, from the league or elsewhere, before he could ever play in a game for them.

He added that the Redskins decided to take a chance on him after consulting with a number of his ex-Alabama teammates on the team, and hoped that being around them would provide him with the "best possible" environment to succeed on and off the field.

[56] In April 2019, an internal NFL investigation found no evidence to support his suspension, with his reinstatement to the team's active roster.

Instead, he was fined two game checks and put into an accountability plan developed in conjunction with the NFLPA and the Redskins, which included counseling, a living arrangement, weekly meetings, and community service to help him avoid future incidents.

[57] During his first practice with the team in May 2019, Foster fell and suffered a torn ACL and LCL and was subsequently placed on injured reserve.

[80] On April 12, he was charged with felony counts of multiple domestic violence, possession of a weapon, and infliction of bodily harm.