Henry James Killilea (June 30, 1863 – January 23, 1929) was an American baseball team owner and attorney. He was one of the founders of baseball's American League. He also played college football and baseball at the University of Michigan.

Killilea was born in Poygan, Wisconsin, in 1863. His parents, Matthew and Mary Killilea emigrated from Ireland in 1849 and established a farm in Winnebago County, Wisconsin. Killilea was raised on the family farm and attended public school in Winneconne, Wisconsin.

After completing grade school, Killilea attended Oshkosh Normal School (now University of Wisconsin–Oshkosh) and then taught school at Clay Banks, Wisconsin.

In the fall of 1882, Killilea enrolled at the University of Michigan, where he studied law. He also played for the Michigan Wolverines baseball team and for the 1883 and 1884 Michigan Wolverines football teams. He played at the forward position on the 1883 team, and as a senior, he played at the center position and was manager of the undefeated 1884 team that outscored its two opponents, 36-10. He graduated from Michigan in 1885.

After graduating from Michigan, Killilea established a law practice in Milwaukee, Wisconsin, in 1885. He specialized in trial practice, with much of his early practice constituting criminal defense. He gained attention for his defense of a school teacher who broke a student's neck. He later worked as counsel to the Chicago, Milwaukee & St. Paul Railroad.

Killilea also had a long association with professional baseball. His younger brother, Matthew Killilea, became the president of the Milwaukee Brewers of the Western League, and Henry became a minority owner.

In the fall of 1899, Killilea and his younger brother, Matthew Killilea, were among the five men who founded baseball's American League, changing the name of the old Western League to the American League in 1900. The other members of the group were Connie Mack, Charles Comiskey, and the leader of the effort, Ban Johnson. Their first meeting was held in Killilea's home at 1616 Grand Avenue in Milwaukee. While Ban Johnson is regarded as the father of the American League, the Sporting Life in July 1904 dubbed Killilea "the godfather of the American League." Killilea's other significant efforts in the early years of the league include the following:

Upon the organization of the American League, Killilea became the majority owner of the Milwaukee Brewers. In 1901, the American League began to operate as a major league which included the Brewers. The 1901 Milwaukee Brewers finished in last place with a 48–89 record. In approximately July 1901, league president Ban Johnson began negotiating with Killilea and his brother, Matthew Killilea, with respect to moving the club out of Milwaukee. In December 1901, the majority of the league's owners decided to eliminate the Milwaukee club and move it to St. Louis as the St. Louis Browns. Displeased with the decision of the owners, Killilea announced his intention to sell his interest in the club. In late January 1902, Killilea and his younger brother, Matthew Killilea, sold their interested in the club to a group in St. Louis.