Per The Athletic's Ken Rosenthal, the Blue Jays will send a minor league player to be named later to Seattle for Walker.
The Blue Jays will be Walker's third team since 2019. He played that year with the Arizona Diamondbacks, but only made one appearance in the final game of the regular season after his recovery from Tommy John surgery in April 2018.
After being nontendered by the Diamondbacks, Walker signed a one-year, $2 million contract with the Mariners in February.
With the Mariners still in the early stages of a rebuild, a player like Walker carried more value to them as a trade piece than a potential long-term asset. His already-low salary looks even more attractive halfway through the season, as teams figure to be more conscious about money with no ticket revenue coming in.
Walker has been able to put together a solid performance thus far in 2020. The 28-year-old has a 4.00 ERA with 25 strikeouts in 27 innings. His fastball velocity isn't quite back to pre-Tommy John levels, but it is averaging a respectable 93 mph, per FanGraphs.
As Walker continues to rebuild his arm strength, the velocity could increase late this season and boost his potential value in free agency.
The Blue Jays are getting Walker as they look to solidify their pitching staff for a potential postseason appearance.
Toronto has been a pleasant surprise so far this season. Its 15-14 record ranks third in the American League East and would be good enough for a wild-card spot in the expanded postseason format.
Hyun-Jin Ryu and Chase Anderson are a solid duo atop the Blue Jays' rotation, but there are significant questions behind them.
Matt Shoemaker, Trent Thornton and Nate Pearson are all on the injured list. Blue Jays manager Charlie Montoyo told reporters on Tuesday that Pearson was diagnosed with a flexor strain in his pitching arm and hopes to play catch this weekend.
Walker isn't an ace who will push Toronto's pitching staff over the top, but he's been much better than anyone the team can call on for depth down the stretch. As long as he can stay healthy for the stretch run, he increases the Blue Jays' chances of making the playoffs for the first time since 2016.