Robert Jermain Thomas (1839 – died c. 31 August 1866) was a Welsh Protestant Christian missionary who served with the London Missionary Society in late Qing Dynasty China and Korea. While serving as a missionary to China, Thomas developed a strong desire to work among the people of Korea. At the time, Korea was closed to foreigners because of the government's fear of foreign influence. Many Koreans had been converted by Catholic priests in the late 1700s, but the government killed 8,000 of those converts in 1866.