Mariann Bienz, Lady Pelham FRS FMedSci (born 21 December 1953, Winterthur) is a Swiss-British molecular biologist based at the UK Medical Research Council Laboratory of Molecular Biology. She has been a member of their Senior Scientific Staff since 1991, was Joint-head of Cell Biology in 2007-08 and has been a Group Leader of Protein and Nucleic Acid Chemistry Division since 2008.