Foster had been away from the team since April, when he was charged with domestic violence, forcefully attempting to prevent a victim from reporting a crime and possessing an assault weapon.
On Wednesday, a California judged dismissed the two felony domestic violence charges against Foster. She also reduced an illegal firearms charge to a misdemeanor, setting a pre-trial date for June 6.
The ruling by the judge came six days after Foster's former girlfriend, Elissa Ennis, testified that she lied about the linebacker assaulting her on Feb. 11.
San Francisco general manager John Lynch said in a statement Wednesday night that the dismissal of the felony charges gave Foster the opportunity to rejoin the team.
"It has been made clear to Reuben that his place on this team is one that must continue to be earned. We will continue to monitor the remaining misdemeanor charge," said Lynch in the statement.
Foster could still face league discipline under the league's personal conduct policy. 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.
"We continue to monitor all developments in the matter which remains under review," NFL spokesman Brian McCarthy said in a statement Wednesday night.
Foster ranked second on the 49ers 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.
The Butkus Award winner at Alabama as the nation's top linebacker, Foster slid to the bottom of the first round as the No. 31 pick in the 2017 NFL Draft because of his character and medical issues (shoulder surgery after his final season with the Crimson Tide).