The deal includes $32 million guaranteed and comes two days before the Seahawks' first practice of training camp.
Nwosu, 26, was the Seahawks' best pass-rusher last season and a bright spot on their underperforming front seven.
In his first season with Seattle after four with the Los Angeles Chargers, Nwosu set career-highs in sacks (9.5), QB hits (26), forced fumbles (four) and tackles for loss (12) while starting all 17 games.
Nwosu was entering the last year of a two-year, $19.055 million deal he signed before last season. At the time, it marked the largest APY of any contract the Seahawks had given to an outside free agent addition under general manager John Schneider and coach Pete Carroll, a distinction now owned by defensive end Dre'Mont Jones, whom Seattle signed for three years and $51.53 million in March.
Nwosu's last Seahawks contract carried a $13.01 million cap charge for 2023 that will likely be lowered with his extension, giving Seattle more breathing room against the NFL's spending limit.
A second-round pick by the Chargers out of USC in 2018, Nwosu has 24.5 sacks in five seasons.