Guillermo Antonio Quiroz (born November 29, 1981 in Maracaibo, Venezuela) is a Major League Baseball catcher for the Seattle Mariners.

Quiroz was signed by the Toronto Blue Jays as a non-drafted free agent in 1998, before his 17th birthday, with a lucrative $1.5 million signing bonus. He progressed through Toronto's system as the team's number-one catching prospect. In addition, Baseball America rated him as the Blue Jays' third best prospect in their 2004 pre-season rankings, and the 35th best prospect in all of baseball.

Defensively, Quiroz has good skills. In 2003 he threw out 45% of base stealers, and was the starting catcher for the World Team in the 2003 All-Star Futures Game.