San Francisco 49ers linebacker Reuben Foster just caught another break ... he won't do any jail time in his illegal weapons case after cutting plea deal with prosecutors.

Foster was initially hit with a misdemeanor possession of an assault weapon charge after cops say they found a SIG Sauer 516 with a high-capacity magazine at the NFL player's home.

He was facing serious prison time if convicted.

But, Foster's legal team got the charge reduced to a misdemeanor and he ultimately cut a deal in which he pled no contest and in exchange got 2 years probation, 232 hours of community service and $235 in fines.

Officials tell TMZ Sports ... as part of his probation, Foster may not possess guns of any kind.