Catherine "Kate" O'Regan (born 17 September 1957) is a former judge of the Constitutional Court of South Africa. From 2013 to 2014 she was a commissioner of the Khayelitsha Commission and is now the inaugural director of the Bonavero Institute of Human Rights at the University of Oxford.

O'Regan was born in Liverpool, England, into a large Catholic family of Irish immigrants. She moved to Cape Town when she was seven. Her mother was a dentist and her father was a doctor.

O'Regan studied at the University of Cape Town from 1975 to 1980, earning a B.A. and LL.B. She was taught briefly by Arthur Chaskalson, who had recently founded the Legal Resources Centre, and ran UCT's legal aid project, working with Mahomed Navsa of the University of the Western Cape. After earning an LLM from the University of Sydney, she returned to South Africa and began her articles of clerkship at Bowman Gilfillan. She stayed on at Bowman for two years under John Brand, specialising in labour law and land rights and representing COSATU, NUM, NUMSA and the Black Sash.

In 1985, O'Regan went to London to do a PhD at the London School of Economics on interdicts restraining strikes. On her return to South Africa in 1988, she worked at the Labour Law Unit and then became an associate professor at the University of Cape Town. She was a founder member of the Law, Race and Gender Research project and the Institute for Development Law at UCT; advised the African National Congress (of which she was a non-active member from 1991) on land claims legislation, working with Geoff Budlender, Aninka Claassens and Derek Hanekom; and served as a trustee of the Legal Resources Centre Trust. She co-edited No Place to Rest: Forced Removals and the Law in South Africa and contributed to A Charter for Social Justice: A Contribution to the South African Bill of Rights Debate.

In 1994, O'Regan was appointed to the newly formed Constitutional Court of South Africa by Nelson Mandela. Aged only 37, O'Regan's appointment was surprising even to her. She and Yvonne Mokgoro were the only female judges on the Court for its first 13 years.

O'Regan's first majority judgment was S v Bhulwana; S v Gwadiso, where the Court for the first time suspended an order of constitutional invalidity. In 1998, she co-authored Fedsure Life Assurance Ltd v Greater Johannesburg Transitional Metropolitan Council, the Court's founding judgment on the rule of law and legality review. The first major judgment of which she was the sole author was Premier, Mpumalanga, still the leading authority on the doctrine of legitimate expectations (on which O'Regan also wrote in Ed-U-College, again in the context of the government's withdrawal of school subsidies). O'Regan's judgment in Dawood v Minister of Home Affairs, delivered in 2000, established for the first time that the right to family life is constitutionally protected and that the conferral of broad discretionary powers on government officials can be unconstitutional. But by far her most-cited contribution to administrative law is her 2004 judgment in Bato Star v Minister of Environmental Affairs and Tourism, which sets out South African law's approach to reasonableness review and judicial deference.

O'Regan's judgment in ACDP v Electoral Commission, dealing with the African Christian Democratic Party's application to contest the 2006 local government elections, introduced the doctrine of substantial compliance into South African law. Her judgment in Richter v Minister of Home Affairs, also on political rights, extended the right to vote to South African citizens living abroad.

O'Regan wrote several judgments on labour law, in which she had specialised as an attorney and academic. She wrote two judgments—one in 1999 and one in 2007—in the ongoing litigation between the South African National Defence Union and the South African National Defence Force, as well as NUMSA v Bader Bop, a judgment dealing with the subject of her PhD thesis: the right to strike. Her 2001 judgment on the relationship between administrative law and labour law, Fredericks v MEC for Education and Training, Eastern Cape, has effectively been overturned—to almost unanimous disapproval by commentators. In Sidumo v Rustenburg Platinum Mines Ltd, O'Regan wrote separately to emphasise, in agreement with the majority judgment of Navsa AJ, her law-clinic colleague of thirty years earlier, that administrative law applies to labour law disputes.