The 49ers agreed to terms with Walker on Wednesday, Matt Barrows of The Athletic reports.
The 29-year-old was a third-round pick of the Lions in 2018 and appeared in 79 games for Detroit, making 43 starts across six seasons. Walker appeared in all 17 regular-season games last year, recording 59 tackles (42 solo), two pass breakups and one sack. Walker gives the 49ers safety depth behind Talanoa Hufanga and Ji'Ayir Brown.