The Mariners will announce a corresponding roster move prior to Monday's contest.
Widely considered the organization's top prospect, Walker was expected to be part of the M's starting rotation at the beginning of the 2014 season before suffering a shoulder injury during spring training.
He began his rehab work with starts in Advanced-A High Desert and Double-A Jackson, and was expected to continue rehabbing in Tacoma but had to be shut down again with shoulder tightness in mid-April.
After his latest recovery, Walker was activated from the disabled list on June 10 and optioned to Tacoma. In six starts for the Rainiers, Walker compiled an ERA of 4.11 and a 2-1 record. In his most recent start, he threw a complete-game shutout against the Oklahoma City RedHawks on Tuesday, giving up just four hits and one walk in a 2-0 victory that took just two hours to complete.
The righty started three games the end of the 2013 season for his only major-league action to date, recording a win in his first start for Seattle, and an ERA of 3.60 in those outings.
Seattle's 2010 first-round pick replaces starter Erasmo Ramirez demoted to Triple-A on Wednesday in the rotation. Ramirez had an ERA of 4.58 in 11 starts in 2014, and lasted longer than six innings in only one of those starts.
The Mariners' pitching has been some of the strongest in all of baseball this season, with starters combining for an 3.45 ERA good for first in the American League. Adding Walker to the mix should give Seattle a midseason boost on the mound and aid the team in its quest for its first playoff appearance since 2001.