The deal marks a return to Seattle for Morales, who led the Mariners in batting average (.277), hits (167), doubles (34) and RBIs (80) last season and set a club record for a switch-hitter with 23 home runs.
"He's a professional hitter," Mariners manager Lloyd McClendon said. "He gives us the opportunity to stretch out our lineup, so to speak. He's a nice fit, switch-hitter, hitting behind Robbie (Cano).
"He gives us options, what we want to do with (Kyle) Seager, where we want to hit him. I like it."
Morales became a free agent after rejecting a $14.1 million qualifying offer from Seattle last November. With his market hindered by the burden of draft-pick compensation, he failed to generate interest until after the first-year player draft passed in early June. On June 8, Morales signed with Minnesota for about $7.4 million -- or the prorated portion of $12 million.
Seattle is competing for a playoff spot despite ranking last in the American League with a .673 OPS and 13th in runs scored with 396. Morales will upgrade Seattle's designated hitter contingent, which has a .569 OPS with eight home runs in 352 at-bats.
Morales was hitting .234 with one home run in 39 games with Minnesota.
"We know what Kendrys is, we know his career as a hitter, we know what he did here and now that he's in a pennant race ... I think we all feel that Kendrys is going to hit his stride and be the Kendrys Morales we all know," Mariners general manager Jack Zduriencik said.
Morales is scheduled to join the Mariners on Friday, at which point the team will make a corresponding move with its 25-man roster.
The Mariners had been linked in trade speculation to Philadelphia's Marlon Byrd and Tampa Bay's Ben Zobrist, among other hitters, before acquiring Morales.
Pryor, 25, was a fifth-round pick by Seattle out of Tennessee Tech in the 2010 first-year player draft. He has a 2.81 ERA and 35 strikeouts in 32 big league innings with the Mariners.
Information from The Associated Press was used in this report.