Sir John Stephen Kós KNZM KC (born 23 January 1959) is a New Zealand judge on the Supreme Court of New Zealand and the former President of the Court of Appeal of New Zealand.^[1]

The son of a Hungarian refugee, Kós was born in Mosgiel, Otago, in 1959 and raised in Wainuiomata.^{[2] [3]} Kós attended Naenae College.^[4] He matriculated at the Victoria University of Wellington to study law in 1976 where he later graduated LLB(Hons) in 1981.^[1]

After graduating from University of Cambridge in 1985 with an LLM, Kós began a career in commercial litigation. In 1985, he became a partner in Perry Wylie & Page, and later a partner in Russell McVeagh in 1988. He went to the independent bar in 2005 and was appointed as a Queen's Counsel in 2007.^[1] He founded Stout Street Chambers, a leading set of barristers, in 2007 with three other QCs.^[5]

He is a Distinguished Fellow at the Victoria University Law School.^[6] He was formerly Pro-Chancellor of Massey University.^[7] He was the 2022/23 James Merralls Fellow at Melbourne University Law School. ^[8]

In April 2011, Kós was appointed to the High Court of New Zealand.^[2] He was elevated to the Court of Appeal in September 2015,^[6] and succeeding Ellen France as President of the court in July 2016.^[9]

As President, he focused on revising the rules of the Court and reforming its processes, introducing a policy to encourage junior counsel to address the Court, and creating three-week sessions of the Permanent Court, followed by two-week circuits by Divisional Courts (with writing time for other judges).  He established regular divisional sittings in Christchurch and Dunedin and more Permanent Court sittings on cases of public interest in Auckland. He also led the project to establish a branch of the Court in the heritage precinct at the Auckland High Court, which opened in 2020. ^[1]

On 8 April 2022, it was announced that Kós would be appointed a judge to the Supreme Court of New Zealand.^[10]

In the 2023 King's Birthday and Coronation Honours, Kós was appointed a Knight Companion of the New Zealand Order of Merit, for services to the judiciary and legal education.^[11]

Justice Kós wrote the judgments in a number of notable cases: in criminal law he wrote the guideline judgment on serious drug offence sentencing (Zhang v R), on mental health deficits as a mitigating factor in sentencing generally (Orchard v R) and judgments in appeals concerning the manslaughter of 3 year-old Moko Rangitoheriri (Shailer v R) and the murder of British backpacker Grace Millane (Kempson v R), in civil and public law he has written leading judgments on fiduciary duties and economic duress (Dold v Murphy) the doctrine of penalties (Wilaci Pty Ltd v Torchlight Fund, 127 Hobson Street Ltd v Honey Bees Preschool Ltd), defamation (Hagaman v Little, Craig v MacGregor) and prospective costs and Beddoe orders in trusts cases (Woodward v Smith, McCallum v McCallum), a judgment in which Kos sought to rein in background evidence admissible in a contract interpretation case did not survive further appeal to the Supreme Court (Bathurst Resources Ltd v L&M Coal Holdings Ltd). In the Supreme Court he dissented in the voting rights case, holding that the 18-year qualification threshold in the Electoral Act was not inconsistent with the Bill of Rights Act (Make it 16 Incorporated v Attorney-General).

Justice Kós has written in three particular areas: