Kenneth John Kaiser (born July 26, 1945) is a former umpire in Major League Baseball who worked in the American League from 1977 to 1999. He spent 13 years in the minor leagues and 23 years in the major leagues, a total of 36 years in professional baseball. Kaiser wore uniform number 21 when the AL adopted them for umpires in 1980.
Kaiser officiated in the World Series in 1987 and 1997, as well as the All-Star Game in 1991. He also umpired in the American League Championship Series in 1980, 1988, 1993 and 1995 (Game 6), and in the American League Division Series in 1981, 1996 and 1997. He also worked the single-game playoff to decide the AL West champion in 1995. On May 6, 1982, he was home plate umpire for Gaylord Perry's 300th career victory. 
During a period in which many umpires were criticized for their weights, Kaiser was the heaviest in the AL; he was listed at 288 pounds from 1983 through 1998. Apart from the unlikelihood that he maintained that precise weight for fifteen years, there are ample reasons to believe that his weight topped 300 pounds for much of that time – although he noted in his 2003 autobiography that he already weighed 260 pounds in high school, the AL listed his weight at 220 pounds from 1977–79 and at 200 from 1980-82 before revising the figure to 288 in 1983. Also, the heaviest umpires during that era in the National League – John McSherry (listed at 328 pounds from 1992–96) and Eric Gregg (listed at 325) – were both later noted as actually exceeding those figures significantly; McSherry, who died on the field of a heart attack in 1996, was believed to have approached 400 pounds, and Gregg also conceded that his weight approached that level. After McSherry's death, Gregg took a two-month leave of absence in which he lost 60 pounds, but was still listed at 305 the following year. The major leagues strongly encouraged other umpires to reduce their weight during that period, though Kaiser's was still listed at 288 for two more years before dropping to 270 in his final season. In a June 2004 column for ESPN's MLB Insider, pitcher Tom Candiotti recalled that Kaiser "wouldn't move three steps to call a play."
Candiotti's assessment of Kaiser was seconded by one of his American League colleagues, Durwood Merrill, who noted that Kaiser usually "marched to the beat of his own drummer" in his autobiography, You're Out and You're Ugly, Too.
Before reaching the major leagues, Kaiser worked as a professional wrestler, wearing a black hood and being known as "The Hatchet Man." In his brief stint as a professional wrestler, he wrestled such famous opponents as William Calhoun.
In 1986, Kaiser was voted the Most Colorful Umpire in the American League in a poll by The Sporting News. He was voted the worst umpire in the AL in a poll of players during 1999 spring training. Later that year, he was among the umpires who submitted their resignations in a failed gambit by the Major League Umpires Association, and he was not among those later re-hired following union negotiations and litigation.
Kaiser's 2003 autobiography, written with the help of author David Fisher, is entitled Planet of the Umps: A Baseball Life From Behind the Plate.
- "The umpire and his discontents. - By Bryan Curtis - Slate Magazine,". Retrieved 2008-12-18.
- "MLB Insider". Retrieved 2008-12-18.
- Charlton, James; Shatzkin, Mike; Holtje, Stephen (1990). The Ballplayers: baseball's ultimate biographical reference. New York: Arbor House/William Morrow. p. 550. ISBN 0-87795-984-6.
- Kaiser, K., & D. Fisher (2003). Planet of The Umps. New York: St. Martin's Press.