Mark Anthony Eichhorn (born November 21, 1960, in San Jose, California) is a retired Major League Baseball pitcher best known for his tenure with the Toronto Blue Jays in the late 1980s and the early 1990s when he often served as a middle reliever/set-up man for All-Star closer, Tom Henke. He was the 1986 American League Rookie Pitcher of the year for the Blue Jays, a campaign in which he set team records for rookie relief in ERA, games, wins, and strikeouts. He would have easily won the AL ERA title had he pitched five more innings to qualify.

Mark Eichhorn made his major league debut for the Blue Jays in 1982 but suffered a severe shoulder injury and would not return to the majors until 1986. The shoulder injury had robbed Eichhorn of most of his fastball velocity and had forced him to turn to an unconventional sidearm motion in which his arm angle was well below the belt when he released the ball. He was notable for an exceptionally low velocity for a major league pitcher but his control and unusual delivery made him an effective player.

Eichhorn played for four different ballclubs during his career: the Toronto Blue Jays (1982, 1986-1988, 1992-1993), Atlanta Braves (1989), California Angels (1990-1992, 1996), and Baltimore Orioles (1994). He made his Major League Baseball debut on August 30, 1982. He would return to the minor leagues after the '82 season and not resurface in the Major Leagues again until 1986. Eichhorn played his final game on September 14, 1996.

Eichhorn is currently the pitching coach for Aptos High School in Aptos, California.