Oscar Littleton Chapman (October 22, 1896 – February 8, 1978) was a political activist in the Democratic Party and served as the U.S. Secretary of the Interior, during the Presidency of Harry S. Truman, from 1949 to 1953.

After helping with Franklin Roosevelt's election in 1932, he was appointed Assistant Secretary of the Department of the Interior.

In 1939, Chapman was an early victim of the House Committee on Un-American Activities, as then-chairman Martin Dies, Jr. published a list of the government employees who were members of a Communist-controlled organization (Chapman was considered a member because there was a record that he had contributed two dollars to the American League for Peace and Democracy, which was raising money for the loyalists during the Spanish Civil War).

At the 1944 Democratic National Convention, Chapman was impressed by Truman sticking to his early agreement to support the current Vice-President Henry A. Wallace.

[4] After end of his service in the Department of the Interior, he practiced law in the firm of Chapman, Duff, and Paul.