Morgan Lewis Martin Sr. (March 31, 1805 – December 10, 1887) was an American lawyer, land speculator, Democratic politician, and one of Wisconsin's founding fathers. He was one of the first lawyers in what is now Wisconsin, and represented the Wisconsin Territory as a delegate to the U.S. House of Representatives during the 29th Congress (1845–1847). He was also president of the constitutional convention that crafted the Constitution of Wisconsin in the winter of 1847–1848, and served in the Council (upper legislative chamber) of the Wisconsin Territory from 1838 to 1844, representing Brown County.

Martin arrived in the area that is now Wisconsin before the Wisconsin Territory was created, when it was still part of the Michigan Territory; he also served in the Michigan Territory legislature in the 1830s. During those early years, he was also instrumental in the development of the city of Milwaukee, Wisconsin; he was a financial partner to Solomon Juneau in his original land claims in what is now Milwaukee, he sketched one of the first maps of the area, assisted Juneau in platting the original Milwaukee village, and assisted in constructing several of the first buildings.

After Wisconsin achieved statehood, he served two years each in the Wisconsin Senate (1858 & 1859) and State Assembly (1855 & 1874). He also served as a Union Army paymaster and U.S. Indian Agent during the 1860s, and served as vice president of the Wisconsin Historical Society and probate judge for Brown County in the last decade of his life.

Martin was a first cousin of Wisconsin Territory governor James Duane Doty. His father, Walter Martin, and grandfather, Adam Martin, served in the New York legislature.

He was born in Martinsburg, New York, and graduated from Hamilton College in 1824. Martin then moved to Detroit, Michigan, in 1826, and studied law, and became an attorney. In May 1827, Martin moved to what is now Green Bay, Wisconsin, on the advice of his cousin, James Duane Doty, to practice law. He formed a partnership with Solomon Juneau and owned much of the land that later became Milwaukee, but sold his share in 1836.

Martin served in the Michigan Territorial Council from 1831 to 1835. At the time, the land that would become Wisconsin was a part of the Michigan Territory. He served in the Wisconsin Territorial Legislature from 1838 to 1844, and served as President of the Territorial Council in 1843. He also served as President at the second Wisconsin Constitutional Convention.

Martin was elected on the Democratic Party ticket as a non-voting member to represent the Wisconsin Territory in the Twenty-ninth Congress, with 6,803 votes to 5,787 for Whig James Collins and 790 for Edward D. Holton of the Liberty Party. He would serve from March 4, 1845, to March 3, 1847.

Martin was a candidate for Governor at the 1848 Wisconsin Democratic Party Convention prior to the state's first gubernatorial election. At the time, the party was split between a faction representing the lead-mining regions of the state, supporting Hiram Barber, and a faction of the eastern counties, supporting Martin. The deadlock between the two factions resulted in a compromise pick—Nelson Dewey.