Kenneth John Kaiser (July 26, 1945 – August 8, 2017) was an American 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 numbers for umpires in 1980.