Mariners Roster & Staff
Transactions
| Date | Transaction |
| 07/30/21 | Arizona Diamondbacks claimed SS Jake Hager off waivers from Seattle Mariners. |
| 07/30/21 | Seattle Mariners signed SS Edwin Arroyo. |
| 07/30/21 | Seattle Mariners recalled RHP Wyatt Mills from Tacoma Rainiers. |
| 07/30/21 | Seattle Mariners sent RHP Vinny Nittoli outright to Tacoma Rainiers. |
| 07/30/21 | Seattle Mariners activated RHP Diego Castillo. |
| 07/29/21 | Seattle Mariners optioned RHP Ryan Weber to Tacoma Rainiers. |
| 07/29/21 | Seattle Mariners activated LHP Tyler Anderson. |
| 07/29/21 | Seattle Mariners placed LHP Hector Santiago on the restricted list. |
| 07/29/21 | Seattle Mariners traded RHP JT Chargois and 3B Austin Shenton to Tampa Bay Rays for RHP Diego Castillo. |
| 07/29/21 | Seattle Mariners traded RHP JT Chargois and 3B Austin Shenton to Tampa Bay Rays for RHP Diego Castillo. |
| 07/29/21 | Seattle Mariners traded RHP JT Chargois and 3B Austin Shenton to Tampa Bay Rays for RHP Diego Castillo. |
| 07/29/21 | Seattle Mariners signed free agent RHP Asher Wojciechowski to a minor league contract. |
| 07/28/21 | 2B Donovan Walton changed number to 8. |
| 07/28/21 | Pittsburgh Pirates traded LHP Tyler Anderson to Seattle Mariners for C Carter Bins and RHP Joaquin Tejada. |
| 07/28/21 | Seattle Mariners designated SS Jake Hager for assignment. |
| 07/28/21 | Pittsburgh Pirates traded LHP Tyler Anderson to Seattle Mariners for C Carter Bins and RHP Joaquin Tejada. |
| 07/28/21 | Pittsburgh Pirates traded LHP Tyler Anderson to Seattle Mariners for C Carter Bins and RHP Joaquin Tejada. |
| 07/27/21 | Seattle Mariners sent CF Jake Fraley on a rehab assignment to Tacoma Rainiers. |
| 07/27/21 | Seattle Mariners signed C Harry Ford. |
| 07/27/21 | Houston Astros traded 3B Abraham Toro, RHP Joe Smith, and to Seattle Mariners for RHP Kendall Graveman and RHP Rafael Montero. |
| 07/27/21 | Houston Astros traded 3B Abraham Toro, RHP Joe Smith, and to Seattle Mariners for RHP Kendall Graveman and RHP Rafael Montero. |
| 07/27/21 | Seattle Mariners activated LHP Hector Santiago. |
| 07/27/21 | Seattle Mariners optioned RHP Darren McCaughan to Tacoma Rainiers. |
| 07/27/21 | Seattle Mariners designated RHP Vinny Nittoli for assignment. |
| 07/27/21 | Houston Astros traded 3B Abraham Toro, RHP Joe Smith, and to Seattle Mariners for RHP Kendall Graveman and RHP Rafael Montero. |
| 07/27/21 | Houston Astros traded 3B Abraham Toro, RHP Joe Smith, and to Seattle Mariners for RHP Kendall Graveman and RHP Rafael Montero. |
| 07/26/21 | Seattle Mariners activated 3B Abraham Toro. |
| 07/26/21 | Seattle Mariners activated 3B Abraham Toro. |
| 07/25/21 | Seattle Mariners optioned 2B Donovan Walton to Tacoma Rainiers. |
| 07/25/21 | Seattle Mariners recalled RHP Ryan Weber from Tacoma Rainiers. |
| 07/23/21 | Seattle Mariners activated RHP Casey Sadler. |
| 07/23/21 | Seattle Mariners designated RHP Rafael Montero for assignment. |
| 07/22/21 | Seattle Mariners signed RHP Michael Morales. |
| 07/22/21 | Seattle Mariners recalled 2B Donovan Walton from Tacoma Rainiers. |
| 07/22/21 | Seattle Mariners optioned RHP Yohan Ramirez to Tacoma Rainiers. |
| 07/21/21 | Seattle Mariners optioned LF Dillon Thomas to Tacoma Rainiers. |
| 07/21/21 | Seattle Mariners selected the contract of RHP Darren McCaughan from Tacoma Rainiers. |
| 07/21/21 | Seattle Mariners sent 3B Wyatt Mathisen outright to Tacoma Rainiers. |
| 07/20/21 | Seattle Mariners signed RHP Riley Davis. |
| 07/20/21 | Seattle Mariners signed free agent LHP Peyton Alford to a minor league contract. |
| 07/20/21 | Seattle Mariners signed OF Spencer Packard. |
| 07/20/21 | Seattle Mariners signed OF Colin Davis. |
| 07/20/21 | Seattle Mariners signed 3B Cole Barr. |
| 07/20/21 | Seattle Mariners signed RHP Bryce Miller. |
| 07/20/21 | Seattle Mariners signed free agent OF Randy Bednar to a minor league contract. |
| 07/20/21 | Seattle Mariners signed C Andy Thomas. |
| 07/20/21 | Seattle Mariners signed SS James Parker. |
| 07/20/21 | Seattle Mariners signed RHP Jimmy Joyce. |
| 07/20/21 | Seattle Mariners signed SS Ben Ramirez. |
| 07/20/21 | Seattle Mariners signed RHP Jimmy Kingsbury. |
| 07/20/21 | Seattle Mariners signed OF Corey Rosier. |
| 07/20/21 | Seattle Mariners signed C Charlie Welch. |
| 07/20/21 | Seattle Mariners signed RHP Bryan Woo. |
| 07/20/21 | Seattle Mariners signed RHP Jordan Jackson. |
| 07/18/21 | Seattle Mariners recalled LF Dillon Thomas from Tacoma Rainiers. |
| 07/18/21 | Seattle Mariners placed CF Jake Fraley on the 10-day injured list. |
| 07/17/21 | RHP Will Vest returned to Detroit Tigers from Seattle Mariners. |
| 07/17/21 | Seattle Mariners signed RHP Andrew Moore. |
| 07/16/21 | Seattle Mariners optioned RHP Ryan Weber to Tacoma Rainiers. |
| 07/16/21 | Seattle Mariners claimed RHP Ryan Weber off waivers from Milwaukee Brewers. |
| 07/16/21 | Seattle Mariners designated 3B Wyatt Mathisen for assignment. |
| 07/16/21 | LHP Hector Santiago roster status changed by Seattle Mariners. |
| 07/13/21 | Seattle Mariners sent RHP Casey Sadler on a rehab assignment to Everett AquaSox. |
| 07/12/21 | Seattle Mariners activated LHP Yusei Kikuchi from the 10-day injured list. |
| 07/12/21 | Seattle Mariners optioned 2B Donovan Walton to Tacoma Rainiers. |
| 07/12/21 | Seattle Mariners designated RHP Will Vest for assignment. |
| 07/11/21 | Seattle Mariners recalled 2B Donovan Walton from Tacoma Rainiers. |
| 07/11/21 | Seattle Mariners placed LHP Yusei Kikuchi on the 10-day injured list. |
| 07/11/21 | Seattle Mariners optioned LF Taylor Trammell to Tacoma Rainiers. |
| 07/11/21 | Seattle Mariners selected the contract of C Cal Raleigh, and from Tacoma Rainiers. |
| 07/09/21 | Seattle Mariners signed free agent 3B Liam Sabino to a minor league contract. |
| 07/09/21 | Seattle Mariners traded RHP Damon Casetta-Stubbs to Cleveland Indians. |
| 07/08/21 | Seattle Mariners signed P Jose Giraldo. |
| 07/07/21 | Seattle Mariners placed LHP Justus Sheffield on the 10-day injured list. Left forearm strain. |
| 07/07/21 | Seattle Mariners activated RHP Erik Swanson. |
| 07/03/21 | Seattle Mariners signed free agent RHP Shane Carle to a minor league contract. |