Sir Leszek Krzysztof Borysiewicz (born 13 April 1951) is a British professor, immunologist and scientific administrator. He served as chairman of Cancer Research UK (2016-2023). Prior to that he was Vice-Chancellor of the University of Cambridge (2010-2017) and chief executive of the Medical Research Council (2007-2010).

Leszek Krzysztof Borysiewicz was born in Cardiff, Wales, to Jan and Zofia (née Wołoszyn) Borysiewicz, ethnic Polish World War II-era refugees (from what is present-day Belarus) who came to Great Britain with the Anders' Army. He speaks fluent Polish. After attending Cardiff High School, Borysiewicz studied at Welsh National School of Medicine of Cardiff University, where he obtained a BSc in anatomy 1972, followed by an MB BCh medical degree in 1975. He received a PhD degree from Imperial College London (then part of the University of London) in 1986 for his thesis on Cell mediated immunity to human cytomegalovirus infection (cytotoxic T cell and natural killer cell mediated lysis of human cytomegalovirus infected cells) supervised by J. G. P. Sissons and Keith Peters. He received a Fellowship of the Royal College of Physicians (FRCP) in 1989. In 2010, Borysiewicz was awarded with an MA degree from the University of Cambridge, upon his inauguration as the new Vice-Chancellor of this University.

Borysiewicz pursued a career in academic medicine at the University of Cambridge, where he was a fellow of Wolfson College, and then as a consultant at Hammersmith Hospital. He headed the Department of Medicine at the University of Wales before joining Imperial College London, where he was promoted to Deputy Rector responsible "for the overall academic and scientific direction of the College," In September 2007, it was reported he would succeed Colin Blakemore as the 9th head of the Medical Research Council, a national organisation that supports medical science with an annual budget of around £500 million.

Borysiewicz was appointed as chairman of Cancer Research UK in November 2016. Cancer Research UK is one of the world's largest fundraising charities and is governed by a council of trustees, led by Borysiewicz. The council's role is to set the charity's strategic direction, uphold its value and governance, and guide, advise and support the chief executive.

Borysiewicz previously chaired the European Research Council Identification Committee (2014-2020); Scientific Advisory Board, Department for International Development, UK Government (2010-2016); and Joint MRC/UK Stem Cell Foundation Scientific Advisory Board (2005-2007). In April 2026, the UK government announced that Sir Leszek was to be the next Chair of UK Research and Innovation from July 2026.

He currently holds several other roles, including as a member of the UK Health Honours Committee, Wales Science and Innovation Council, as a non-executive director at 52 North Health, and Chair of Diamond Light Source, the UK's national synchrotron facility.

Borysiewicz's research focuses on viral immunology, infectious disease, and viral-induced cancer. His work in vaccines included Europe's first trial of a vaccine for human papillomavirus to treat cervical cancer, research conducted at Cardiff University. He has co-authored and co-edited a number of books on these subjects, including Vaccinations.

Borysiewicz was knighted by Queen Elizabeth II in the 2001 New Year Honours List for services to Medical Research and Education. He was appointed a Deputy Lieutenant of Cambridgeshire (DL) in 2012. In October 2018, he was awarded the Grand Cross of the Order of Merit of the Republic of Poland, the highest-ranked Polish order of merit awarded to foreigners or Poles resident abroad for their services to Poland. He collected it during a ceremony at the Polish Embassy in London in late April 2019. He was appointed a Knight Grand Cross of the Order of the British Empire (GBE) by King Charles III in the 2025 New Year Honours List for services to Cancer Research, to Clinical Research, to Medicine and to Charities.