Olaus Magnus Friedrich Erdmann Henrici, FRS (9 March 1840, Meldorf, Duchy of Holstein – 10 August 1918, Chandler's Ford, Hampshire, England) was a German mathematician who became a professor in London.

After three years as an apprentice in engineering, Henrici entered Karlsruhe Polytechnium where he came under the influence of Alfred Clebsch who encouraged him in mathematics.

In 1869 Hesse introduced him to J. J. Sylvester who in turn brought him into contact with Arthur Cayley, William Kingdon Clifford, and Thomas Archer Hirst.

In 1884 he moved to Central Technical College where he directed a Laboratory of Mechanics which included calculating machines, planimeters, moment integrators, and a harmonic analyzer.

[2] Henrici was impressed by the work of Robert Stawell Ball in screw theory as presented in a German textbook by Gravelius.