David Davis III (March 9, 1815 - June 26, 1886) was a United States Senator from Illinois and associate justice of the Supreme Court of the United States.

Davis, a cousin of Henry Winter Davis, was born to a wealthy family in Cecil County, Maryland, where he attended the public schools. After graduating from Kenyon College in Gambier, Ohio, in 1832 he went on to study law at Yale University, and upon his graduation from Yale in 1835, moved to Bloomington, Illinois, to practice law. From 1848-1862, Davis presided over the local judicial circuit, the same circuit where attorney Abraham Lincoln was practicing. Davis assisted Lincoln in Lincoln's presidential campaign in 1860.

In 1862, President Lincoln appointed Davis to the U.S. Supreme Court, where he became famous for writing one of the most profound decisions in the Supreme Court history, Ex Parte Milligan (1866). In that decision, the court set aside the death sentence imposed during the Civil War by a military commission upon a civilian, Lambdin P. Milligan. Milligan had been found guilty of inciting insurrection. The Supreme Court held that since the civil courts were operative, the trial of a civilian by a military tribunal was unconstitutional. The opinion denouced arbitrary military power, effectively becoming one of the bulwarks of held notions of American civil liberty.

After refusing calls to become Chief Justice of the Supreme Court, Davis, a registered independent, was nominated for President by the Labor Reform Convention in 1872, but withdrew when he failed to receive the Liberal Republican Party nomination. He resigned from the Supreme Court in 1877 to serve as a U.S. Senator from Illinois until 1883.