Lydia Mary Fay (1804 - October 5, 1878) was a 19th-century American missionary, educator, writer, and translator. She was the "first North American single-woman missionary to China", and one of the band of women that laid broad and deep foundations in the early days of missionary work in the Chinese empire. For nineteen months, Fay worked on revising the Syllabic Dictionary manuscript. She contributed to magazines and papers, and had exceptional translation skills from Chinese language into English.