Rienzi Melville Johnston (September 9, 1849 – February 28, 1926) was an American journalist and politician.
He edited the Houston Post from 1885 to 1919, and served a 29-day term in the United States Senate in January 1913 after the resignation of Joseph Weldon Bailey.
His term remains the second shortest in Senate history.
Johnston was a member of the Texas Senate from 1917 to 1920, and also its President pro tempore from 1918.