William Joseph Klem (born Klimm; February 22, 1874 – September 16, 1951), known as "the Old Arbitrator", was an American baseball umpire who worked in Major League Baseball from 1905 to 1941, spending his entire career in the National League (NL).

Of the 16 major league teams in existence during his career, all but one—the St. Louis Browns, who would not win a pennant until 1944—appeared in a World Series that he officiated; the only other teams which did not win a championship with Klem on the field were the Brooklyn Dodgers, Philadelphia Phillies (neither of which won a title during Klem's lifetime) and the Detroit Tigers.

[6] He called balls and strikes in five no-hitters, an NL record later tied by Harry Wendelstedt.

He was also the home plate umpire on September 16, 1924, when Jim Bottomley of the St. Louis Cardinals had a record 12 runs batted in.

The catcher had covered the photo with dirt and waited for Klem to brush off home plate.

"[2] Late in his life, Klem stated in interviews that he had originated the use of hand signals for umpiring calls.

Recent research does not yield a clear answer to the origin of hand signals, with credit often going to umpire Cy Rigler.

[10] By 1940, Klem had retired and had been replaced by future Hall of Fame umpire Al Barlick.

Klem was widely respected for bringing dignity and professionalism to umpiring, as well as for his high skill and good judgment.

[2] Klem died on September 16, 1951, at age 77, at Doctors Hospital in Coral Gables, Florida.

In 1962, the Houston chapter of the Baseball Writers' Association of America established the Bill Klem Award to honor outstanding NL umpires.