A Regius Professor is a university professor who has, or originally had, royal patronage or appointment. They are a unique feature of academia in the United Kingdom and Ireland. The first Regius Professorship was in the field of medicine, and founded by the Scottish King James IV at the University of Aberdeen in 1497. Regius chairs have since been instituted in various universities, in disciplines judged to be fundamental and for which there is a continuing and significant need. Each was established by an English, Scottish, or British monarch, and following proper advertisement and interview through the offices of the university and the national government, the current monarch still appoints the professor (except for those at the University of Dublin in Ireland, which left the United Kingdom in 1922). This royal imprimatur, and the relative rarity of these professorships, means a Regius chair is prestigious and highly sought-after.

Regius Professors are traditionally addressed as "Regius" and not "Professor". The University of Glasgow currently has the highest number of extant Regius chairs, at fourteen.

Traditionally, Regius Chairs only existed in the seven ancient universities of the UK and Ireland. In October 2012 it was announced that Queen Elizabeth II would create up to six new Regius Professorships, to be announced in early 2013, to mark her Diamond Jubilee. In January 2013 the full list was announced, comprising twelve new chairs, probably the largest number ever created in one year, and more than created in most centuries. In July 2015 it was announced that further Regius Professorships would be created to mark the Queen's 90th birthday.