Robert S. Lewis (August 15, 1856 – May 23, 1956) was a North Dakota Republican Party politician who served as the seventh Lieutenant Governor of North Dakota under Governor John Burke. Lewis also served in the North Dakota Senate from 1901 to 1905. Lewis died in 1956 at the age of 99, only few months shy of his 100th birthday, making him the oldest statewide officer in the state's history.