Lyman Trumbull (October 12, 1813 - June 25, 1896) was a United States Senator from Illinois during the American Civil War.

Trumbull was born in Colchester, Connecticut. He attended Bacon Academy and was a school teacher from 1829 to 1833. After studying law, he was admitted to the bar and practiced in Greenville, Georgia until moving to Belleville, Illinois in 1837.

By 1840, he was serving in the Illinois State House and was Secretary of State from 1841 to 1843. From 1848 to 1853, he was a justice on the state Supreme Court. Although elected to the United States House of Representatives in 1854, he was elected to serve in the U.S. Senate before he could take his seat. He served from 1855 through 1873, during which time he claimed party affiliations with the Democrats, the Republicans, the Liberal Republicans, and finally the Democrats again.

During President Andrew Johnson's impeachment trial, Trumbull broke party ranks, along with six other Republican senators, and in a courageous act of political suicide, voted for acquital. These seven Republican senators were disturbed by how the proceedings had been manipulated in order to give a one-sided presentation of the evidence. Senators William Pitt Fessenden, Joseph S. Fowler, James W. Grimes, John B. Henderson, Lyman Trumbull, Peter G. Van Winkle , and Edmund G. Ross of Kansas, who provided the decisive vote , defied their party and public opinion and voted against impeachment.

As chairman of the Judiciary Committee (1861-1872), he co-authored the Thirteenth Amendment, which prohibited all kinds of slavery in the United States.

In 1873, he set up a law practice in Chicago and remained in private practice except for a brief period when he ran an unsuccessful campaign for governor (as a Democrat) in 1880.

During his explorations in the west John Wesley Powell named Mt. Trumbull (and now the Mt. Trumbull Wilderness) in northwestern Arizona after the senator.

The Lyman Trumbull House is a National Historic Landmark.