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.

Matthew D. Lagan
Member of the U.S. House of Representatives from Louisiana's 2nd district
In office March 4, 1891 – March 3, 1893
Preceded by	Hamilton D. Coleman
Succeeded by	Robert C. Davey
In office March 4, 1887 – March 3, 1889
Preceded by	Nathaniel D. Wallace
Succeeded by	Hamilton D. Coleman
Personal details
Born	Matthew Diamond Lagan (1829-06-20)June 20, 1829 Maghera, Ireland
Died	April 8, 1901(1901-04-08) (aged 71) New Orleans, Louisiana
Resting place	Metairie Cemetery, New Orleans
Political party	Democratic

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.

Lagan died in New Orleans, Louisiana on April 8, 1901, and is buried in Metairie Cemetery.