His deal will keep him in Jacksonville through 2023.
Jack, 23, wound up being selected by the Jaguars with the No. 36 overall pick in the 2016 NFL Draft after they traded up to acquire him. He was entering the final year of his four-year, $6.3 million rookie contract.
In 2018, Jack appeared in all 16 games and recorded 107 total tackles, one tackle for loss, 2.5 sacks, one interception, one forced fumble, and one fumble recovery. Pro Football Focus had him rated as the No. 33 overall linebacker out of 92 qualifying players.