Lillias Horton Underwood (June 21, 1851 – October 29, 1921), born Lillias Stirling Horton, was an American physician and Presbyterian missionary in Korea, alongside her husband Horace Grant Underwood.

[3][4] Horton arrived in Korea as a medical missionary in 1888, under the auspices of the Presbyterian Board of Foreign Missions.

[3] Underwood wrote about her experiences abroad in several books, including Fifteen Years Among the Topknots (1904, 1908),[8] With Tommy Tompkins in Korea (1905),[9] and Underwood of Korea (1918), a biography of her husband.

Underwood was a widow when she died in 1921, aged 70, in Korea, from the effects of tropical sprue.

[13][14] Underwood's grandson Horace Grant Underwood II (1917-2004) was a Korean language specialist with the United States Navy during World War II and the Korean War.