Sir Stephen Patrick O'Rahilly (born 1 April 1958) is an Irish-British physician and scientist known for his research into the molecular pathogenesis of human obesity, insulin resistance and related metabolic and endocrine disorders.

He is also the scientific Director of the National Institute for Health and Care Research (NIHR) Cambridge Biomedical Research Centre, Associate Faculty at the Wellcome Trust Sanger Institute and honorary consultant physician at Addenbrooke's Hospital, Cambridge.

Notable Cambridge scientists with whom O'Rahilly has shared paper authorship include Krishna Chatterjee, David Dunger, Sadaf Farooqi, Nita Forouhi, Antonio Vidal-Puig, Nick Wareham, and Giles Yeo.

His nomination reads: Stephen O'Rahilly has made major contributions to the understanding of the molecular mechanisms underlying human disorders of energy balance and metabolism.

His work first established that mutations in single genes could result in severe human obesity and that these defects largely acted through disruption of central satiety mechanisms.