Mark Thomas Teahen (born September 6, 1981, in Redlands, California) is an American professional baseball infielder for the Toronto Blue Jays of Major League Baseball.
He graduated from Yucaipa High School in 1999 and attended St. Mary's College in California. Teahen was drafted 39th overall in the 2002 draft by the Oakland Athletics with a sandwich pick between the first and second rounds. Teahen was one of the featured players in the book Moneyball, which claimed that Teahen had the potential to become the next Jason Giambi.
Oakland traded him to Kansas City in 2004, along with Mike Wood, as part of a three-way trade that sent Carlos Beltran to the Houston Astros, Octavio Dotel from the Astros to the Athletics, and John Buck from the Astros to the Royals. Although he had been one of the most highly regarded prospects in the Oakland organization, he became available when the Athletics were able to sign Eric Chavez to a long-term contract.