Santiago's 10-game suspension was upheld Thursday, Jeff Passan of ESPN.com reports.
Santiago was ejected from a game in late June for having a foreign substance on his glove, but he was able to pitch while appealing his suspension. However, he'll serve the full length of his suspension following the All-Star break, and the Mariners will have to play with a shorthanded roster for the next 10 games. The southpaw will be eligible to return July 27 against the Astros.