Isabella Caroline McMillen, AO, FAHMS (born 23 September 1954) is an Australian medical and health academic and the incumbent Chief Scientist of South Australia since October 2018. She is a Fellow of the Australian Academy of Health and Medical Sciences, a Fellow of the Royal Society of New South Wales and a Bragg Member of the Royal Institution of Australia. She is a Director of Compass Housing Services Co Ltd, the South Australian Health and Medical Research Institute, the Australian Science Media Centre and a member of the Council of the University of South Australia.