A US judge also ordered Smith to pay $2,000 in fines, according to the San Jose Mercury News.
The court on Friday reduced three felony counts of possessing illegal assault rifles against Smith to misdemeanors.
Under the terms of his probation, the 24-year-old Smith is barred from drinking alcohol for three years. He also cannot own a gun for three years.
The talented but troubled National Football League linebacker has had numerous run-ins with police.
Smith also took a leave of absence from the 49ers last year to seek help for a substance abuse problem. He has been one of the NFL's elite pass rushers since being drafted seventh overall by the 49ers in 2011.