Matthew Diamond Lagan (June 20, 1829 – April 8, 1901) was a U.S. Representative from Louisiana's 2nd congressional district from 1887 to 1889 and 1891 to 1893. He was a Democrat. Lagan was born in Maghera, County Londonderry, Ireland, and immigrated to the United States at age 14. In New Orleans, he fitted out vessels for the Confederate Navy, and served in the Confederate Navy himself. He was elected to terms on the New Orleans City Council in 1867 and 1882. After serving his first term in Congress, he chose not to run for reelection, but then ran in 1890 and defeated his successor, Hamilton D. Coleman.