| Werner Boy | |
| Birth Date: | 4 May 1879 |
| Birth Place: | Barmen, Rhine Province, German Empire |
| Death Place: | near Vitry-le-François, France |
| Nationality: | German |
| Field: | Mathematician |
| Alma Mater: | University of Göttingen |
| Doctoral Advisor: | David Hilbert |
| Known For: | Topology |
Werner Boy (pronounced as /de/; 4 May 1879 − 6 September 1914) was a German mathematician. He was the discoverer and eponym of Boy's surface—a three-dimensional projection of the real projective plane without singularities, the first of its kind. He discovered it in 1901 after his thesis adviser, David Hilbert, asked him to prove that it was not possible to immerse the real projective plane in three-dimensional space. Boy sketched several models of the surface, and discovered that it could have 3-fold rotational symmetry, but was unable to find a parametric model for the surface. It was not until 1978 that Bernard Morin found the first parametrisation, with the aid of computers.
After completing his dissertation, Boy worked as a high school teacher in Krefeld, Germany. He later returned to his birth town of Barmen (today Wuppertal) to teach. He died as a soldier in the first weeks of World War I on 6 September 1914.