Sydenham Benoni Alexander (8 December 1840 - 14 June 1921) was a Democratic U.S. Congressman from North Carolina between 1891 and 1895.

Alexander, born near Charlotte, North Carolina in 1840, attended preparatory schools in Rocky River and Wadesboro and graduated from the University of North Carolina at Chapel Hill in 1860.

At the outbreak of the American Civil War in 1861, he enlisted in the Confederate Army as a private in the First Regiment, North Carolina Volunteer Infantry. He was elected captain of Company K, Forty-second North Carolina Regiment, in June 1862. He detached from his company in 1864 and served as inspector general on the staff of Maj. Gen. Robert F. Hoke ,

After the Civil War, Alexander was a member of the North Carolina Senate in 1879, 1883, 1885, 1887, where he was instrumental in the establishment of the North Carolina College of Agriculture and Mechanic Arts (later North Carolina State University; Alexander served on its board of trustees), and was president of the North Carolina Railroad .

A Democrat, Alexander was elected to the U.S. House in 1890 and served in the 52nd and 53rd Congresses. He did not run for a third term in 1894, but retired to his "Enderly Plantation" estate in Mecklenburg County. He served one additional term in the North Carolina Senate in 1901.

Alexander finally moved to Charlotte in 1906 and died there in 1921; he is buried in Elmwood Cemetery.