The team announced the signing Wednesday but did not disclose financial terms.
The Jaguars' biggest issue defensively is their lack of quality pass rush, and adding Key is a solid first step in the process. Key had the best year of his career in 2021 with the San Francisco 49ers (6.5 sacks) while playing only 35% of the defensive snaps. However, he has just 9.5 sacks in 54 career games and had just three sacks in his first 37 games. Key does give the Jaguars an upgrade from K'Lavon Chaisson, the 20th overall pick in 2020 who has just two sacks in 31 career games, at the other outside linebacker spot opposite Josh Allen.
The Jaguars sacked opposing quarterbacks 32 times last season -- only eight teams had fewer. Opposing quarterbacks completed a league-high 64.9% of their passes and the Jaguars intercepted just seven passes. Allen led the Jaguars with 7.5 sacks and Dawuane Smoot had six last season.
The Jaguars have the first overall pick in the upcoming NFL draft, and Michigan pass-rusher Aidan Hutchinson is one of the players they're considering.
Signed as a free agent after the Raiders released him, Key flourished under Niners defensive line coach Kris Kocurek in 2021.
San Francisco moved Key all over the defensive line, and he found success rushing the passer from the interior. Key's 6.5 sacks were second on the team only to Nick Bosa, who had 15.5.
Key, 25, added 22 tackles, five tackles for loss and 17 quarterback hits. He was originally a third-round pick of the Raiders in 2018.
ESPN's Michael DiRocco and Nick Wagoner contributed to this report.