Caspar Wessel (June 8, 1745, Vestby – March 25, 1818, Copenhagen) was a Norwegian-Danish mathematician and cartographer. In 1799, Wessel was the first person to describe the geometrical interpretation of complex numbers as points in the complex plane. He was the younger brother of poet and playwright Johan Herman Wessel.

Wessel was born in Jonsrud, Vestby, Akershus, Norway. In 1763, having completed secondary school, he went to Denmark for further studies. He attended the University of Copenhagen and acquired the degree of candidatus juris in 1778. From 1794, however, he was employed as a surveyor (from 1798 as Royal inspector of Surveying).

It was the mathematical aspect of surveying that led him to exploring the geometrical significance of complex numbers. His fundamental paper, Om directionens analytiske betegning, was published in 1799 by the Royal Danish Academy of Sciences and Letters. Since it was in Danish, it passed almost unnoticed, and the same results were later independently rediscovered by Argand in 1806 and Gauss in 1831.