Michael Eugene Reilly is a former Major League Baseball umpire who worked in the American League from 1977 to 1999 and throughout both major leagues from 2000 to 2010. Upon Ed Montague's retirement in February 2010, Reilly became Major League Baseball's senior umpire; his 4,362 career games ranked ninth in major league history entering the 2010 season. He retired on February 23, 2011, along with fellow umpires Jerry Crawford and Chuck Meriwether.

In baseball, the umpire is the person charged with officiating the game, including beginning and ending the game, enforcing the rules of the game and the grounds, making judgment calls on plays, and handling the disciplinary actions. The term is often shortened to the colloquial form ump. They are also sometimes addressed as blue at lower levels due to the common color of the uniform worn by umpires. In professional baseball, the term blue is seldom used by players or managers, who instead call the umpire by name. Although games were often officiated by a sole umpire in the formative years of the sport, since the turn of the 20th century, officiating has been commonly divided among several umpires, who form the umpiring crew. The position is analogous to that of a referee in many other sports.