The 23-year-old UCLA product had one year left on his rookie deal after the Jags selected him in the second round of the 2016 draft. The extension links him to the organization through the 2023 season.
Jack is coming off the most productive campaign of his career. He registered 107 total tackles, 2.5 sacks, one forced fumble and an interception he returned for a touchdown in his third NFL season.
In June, the Arizona native said he was willing to play out the final year of his contract if the sides weren't able to reach a deal.
"Obviously in my position, Telvin [Smith] is gone, so that's 120 tackles unaccounted for, and I want at least 50 of those so I can get 150 tackles to create some leverage for myself," Jack told reporters. "My job is to play Mike [linebacker]. I have to know way too much to miss [minicamp]. I've got to run the defense."
Perhaps sensing Jack was in line for a monster statistical season, the Jaguars opted to lock him into a long-term contract now before the price potentially went up following 2019.
He's expected to anchor a Jacksonville defense that ranked fifth in yards allowed last season.
Expectations are back on the rise for the Jags after they signed quarterback Nick Foles to reinvigorate an offense that struggled mightily in 2018 leading to a disappointing 5-11 record after reaching the AFC Championship Game the previous season.