Smith, 31, spent the 2020 season in Dallas after missing the previous four seasons amid myriad off-field issues. He recorded 48 tackles and five sacks, along with 20 quarterback pressures, in his first football in a half-decade.
"It's inspirational to see a man like Aldon Smith have the comeback here that he had," Cowboys owner Jerry Jones told reporters after the season. "He played some outstanding football for us. He certainly was more positive than not, and I think he can take this year and build on it, and we want him to build on it to the benefit of the Cowboys."
Smith began his career in San Francisco and emerged as one of the most dominant pass-rushers in the sport. He recorded 14 sacks as a rookie in 2011 before making the All-Pro and Pro Bowl teams in 2012, recording 19.5 sacks and helping lead the 49ers to a Super Bowl XLVII appearance.
The off-field troubles began the following season when Smith checked himself into a rehabilitation clinic after being arrested and charged with driving under the influence. He would not play a full season again until 2020, serving a nine-game suspension in 2014 for violations of the league's substance abuse and personal conduct policies before being released in 2015 after a third DUI charge.
The Raiders signed Smith ahead of the 2015 season, but after nine games, the NFL suspended him for a year for violating the league's personal conduct policy. The NFL did not reinstate Smith in either the 2016 or 2017 season, and the Raiders released him in 2018 after an arrest on allegations of domestic violence.
Smith spent the next two years without a team, dealing with alcoholism and becoming homeless at one point.
"I went through some dark times," Smith told Jonny Auping of the Cowboys' official website. "When you're in dark places, it's hard to see the light. I know that might sound cliche, but it really is tough for you to see that there's a possibility that anything could get better."
Seattle could use his production after ranking 31st in the NFL against the pass last season. Safety Jamal Adams was the team's best pass-rusher, and no front-seven player had more than seven sacks last season.
If he can stay on the field, Smith could help the Seahawks defend their NFC West title.