Sir Peter Julius Lachmann FRS FMedSci (23 December 1931 – 26 December 2020) was a British immunologist, specialising in the study of the complement system. He was emeritus Sheila Joan Smith Professor of Immunology at the University of Cambridge, a fellow of Christ's College, Cambridge and honorary fellow of Trinity College, Cambridge and of Imperial College. He was knighted for service to medical science in 2002.