Dame Isobel Baillie, DBE (9 March 1895 – 24 September 1983), née Isabella Douglas Baillie, was a Scottish soprano. She made a local success in Manchester, where she was brought up, and in 1923 made a successful London debut. Her career, encouraged by the conductor Sir Hamilton Harty, quickly developed, with breaks in the first years for vocal study in Milan. Baillie's career was almost wholly as a concert singer: she only once acted in an opera production on stage. She was associated above all with oratorio, becoming well known for her many performances in Handel's Messiah, Haydn's The Creation, Mendelssohn's Elijah and the choral works of Elgar.
During a long career, Baillie sang in complete recordings of Messiah, Elijah, Beethoven's Missa solemnis and Ninth Symphony. In the 1940s she formed a friendship with the contralto Kathleen Ferrier, with whom she appeared frequently in concert and made several recordings of duets. She took part in 19 annual Three Choirs Festivals from 1929. She made her American debut in 1933 and between then and her retirement from the concert platform in the mid 1950s she sang in the US, New Zealand, the Far East and Africa as well as in Europe.
In her later years, Baillie was a teacher in the Royal College of Music, Royal Manchester School of Music and Cornell University.