Mary Deyo (January 8, 1858 – December 15, 1932) was an American teacher and Christian missionary in Yokohama, Ueda, and Morioka in Japan, from 1888 to 1905.

She was a teacher at Ferris Seminary, a girls' school in Yokohama, from 1888 to 1894, and worked as an evangelist in Ueda after 1895.

[2][3] She also worked with fellow American missionary Mary Leila Winn in Morioka from 1903 to 1905.

[4][5][6] Her work was supported by the Women's Board of Foreign Missions of the Reformed Church in America, and by the King's Daughters, a churchwomen's organization.

Her papers, including correspondence from her years in Yokohama, are in the collection of Huguenot Historical Society and the Elting Memorial Library in New Paltz.