Lecompton Constitution, a proposed constitution under which the Territory of Kansas would have entered the Union as a slave state. The constitution was drawn up at Lecompton, the territorial capital, in 1857. After much controversy it was rejected by Congress.

The constitutional convention at Lecompton was dominated by proslavery Kansans; free-state, or antislavery, voters had boycotted the election to pick delegates because of fraudulent procedures. The delegates wrote a constitution permitting slavery; they gave the voters only once choice—between a provision that would allow more slaves to be brought in and one that would not. In the election, which was marked by fraud and boycotted by the antislavery voters, the first proposition won. The U.S. House of Representatives rejected the constitution and in 1858 it was voted down decisively by Kansans in a new referendum.