Clayton Grant Mortensen (born April 10, 1985) is an American former professional baseball pitcher and current coach.

He played in Major League Baseball (MLB) for the St. Louis Cardinals, Oakland Athletics, Colorado Rockies, and Boston Red Sox.

After graduating from Madison High School, Mortensen went to Treasure Valley Community College, then Gonzaga University.

[4] On July 24, 2009, Mortensen was traded to the Oakland Athletics along with Brett Wallace and Shane Peterson for Matt Holliday.

[5] Mortensen was designated for assignment in January 2011 and was eventually traded to the Colorado Rockies for minor league pitcher Ethan Hollingsworth.

[9] On August 27, 2013, the Red Sox traded Mortensen to the Kansas City Royals for Quintin Berry.

In 2014, Mortensen made 16 appearances (15 starts) for the Triple-A Omaha Storm Chasers, posting a 5-4 record and 4.74 ERA with 62 strikeouts in 76 innings pitched.

In 2015, Mortensen returned to Omaha, pitching in 24 games (starting 17) and logging a 7-9 record and 5.40 ERA with 81 strikeouts in 106+2⁄3 innings of work.

[12] On April 4, 2017, Mortensen signed a minor league contract with the Miami Marlins, and was assigned to the Triple-A New Orleans Baby Cakes.

[14] On January 20, 2018, Mortensen signed a minor league deal with the Kansas City Royals.