Francis Butler Loomis (July 27, 1861 – August 4, 1948) served as the United States Ambassador to Venezuela from 1897 to 1901 and the United States Ambassador to Portugal from 1901 to 1902. He was the United States Assistant Secretary of State from 1903 to 1905 when he was appointed as the acting United States Secretary of State. His son was Major general Francis B. Loomis Jr.