The pact is worth $2 million, sources told ESPN's Jeff Passan. The 27-year-old can reportedly earn up to $3 million based on incentives, Passan added.
Once a heralded prospect, Walker tossed 357 innings for the Mariners before he was traded to the Arizona Diamondbacks ahead of the 2017 season.
He only made 32 starts for the D-Backs in three seasons, missing almost all of 2018 and 2019 because of a lengthy recovery from Tommy John surgery.
"We're excited to bring Taijuan back to Seattle," Mariners general manager Jerry Dipoto said in a statement. "We believe our opportunity and his skill set are a great match and (we) know that, if healthy, he can be an impact major-league starter for us."
Across seven seasons in the majors, Walker owns a 31-31 record with a 3.95 ERA and 4.21 FIP in 528 1/3 innings.