Casey Paul Fossum (born January 6, 1978) is an American professional baseball pitcher playing for the Baltimore Orioles. Previously, he played for the Boston Red Sox (2001–2003), Arizona Diamondbacks (2004), Tampa Bay Devil Rays (2005–2007), Detroit Tigers (2008), and New York Mets (2009) of Major League Baseball, and the Hanshin Tigers (2010) of Nippon Professional Baseball. He bats and throws left-handed.
Fossum was born in Cherry Hill, New Jersey. He attended Midway High School in Waco, Texas. He then attended Texas A&M University, leading the Aggies to the 1999 College World Series in Omaha, Nebraska. He earned All-Big 12 honors in both 1998 and 1999, highlighted by a 12–7 record and 3.64 earned run average (ERA) during his junior campaign, establishing a single-season school mark with 162 strikeouts. Fossum also won the C. E. "Pat" Olsen Outstanding Pitcher Award in 1999 and was a second team All-American in 1998.
Fossum was selected by the Boston Red Sox in the 1999 June draft as the 18th pick in the first supplemental round (48th overall), and started his major league career in 2001.