John Forrest Dillon (December 25, 1831 – May 6, 1914) was an American jurist who served on federal and Iowa state courts. He authored a highly influential treatise on the power of states over municipal governments.
Dillon was born in Northampton, Montgomery County, New York (now part of Fulton County, New York). He studied medicine at the University of Iowa at the age of 19. Shortly after beginning his medical practice, he abandoned it to read law, and was admitted to the Iowa bar in 1852. He worked in private practice until he was elected Scott County prosecutor in 1853, and then to a judgeship in Iowa's 7th Judicial Circuit in 1858. He was elevated to the Iowa Supreme Court, serving from January 1, 1864, until he resigned December 31, 1869. For two years of this period he was Chief Justice. in 1869, President Grant appointed him to the United States Circuit Court, which became the Eighth Circuit.
While on the federal bench, Dillon wrote Municipal Corporations (1872), one of the earliest systematic studies of the subject. He also authored Removal of Cases from State Courts to Federal Courts and Municipal Bonds, both in 1876. On February 17, 1876, during the Whiskey Ring graft prosecutions, Justice Dillon ruled Ulysses S. Grant's deposition for Orville E. Babcock was admissible in court. After leaving the Circuit Court, Dillon was a professor at Columbia Law School from 1879 until 1882, where he taught real estate and equity. He taught at Yale Law School from 1891 until 1892, during which time he also served as the president of the American Bar Association. Dillon then returned to private practice until his death in 1914 in New York City.