Ida May Routh (12 July 1934 – 1 June 2022) was a British-American costume designer. Routh was born in India, where she grew up in Jabalpur.

She relocated to England for her education, graduating from the Saint Martin's School of Art. She first worked as a fashion illustrator, with her work appearing in the magazines Vogue and Elle. In 1969, she was offered a position by her former boyfriend Brian Duffy as an assistant to costume designer Anthony Mendleson for the film Oh! What a Lovely War (1969).