John Thomas Alsop Jr. (August 10, 1874 – May 12, 1958) was an American politician. He served as Mayor of Jacksonville, Florida, from 1923 to 1937, and again from 1941 to 1945. His eighteen-year, seven term stint in the mayor's office is the longest in the city's history. Jacksonville's Main Street Bridge, officially the John T. Alsop Jr. Bridge, is named for him.