Derrick Norman Lehmer (27 July 1867 – 8 September 1938) was an American mathematician and number theorist.

He was educated at the University of Nebraska, obtaining a bachelor's degree in 1893 and a master's in 1896.

Lehmer was awarded his Ph.D. from the University of Chicago in 1900 for a thesis, Asymptotic Evaluation of Certain Totient-Sums, under the supervision of E. H. Moore.

He was appointed instructor in mathematics at the University of California, Berkeley, in 1900 and married Clara Eunice Mitchell on 12 July 1900 in Decatur, Illinois.

In 1903, he presented a factorization of Jevons's number (8,616,460,799) at the San Francisco Section of the American Mathematical Society on December 19, 1903.