John Arnold Heydler (July 10, 1869 – April 18, 1956) was an American executive in Major League Baseball.

Born in La Fargeville, New York, Heydler was a printer's apprentice as a young man, and he eventually worked at the U.S. Government Printing Office.

Heydler's work caused him to record much of the league's early history, and he became an advocate for new ways to measure player accomplishments; for example, he was a strong supporter of recording runs batted in for batters and he began computing earned run averages for pitchers.

In December 1928, Heydler proposed permitting a tenth player to bat in place of the pitcher[1] – a rule which came about with the creation of the designated hitter in 1973.

After retiring as league president, he served as NL chairman until his death in San Diego, California, in 1956, aged 86.