KU Leuven, officially known as Katholieke Universiteit Leuven, is a research university in the city of Leuven, Belgium. It conducts teaching, research, and services in computer science, engineering, natural sciences, humanities, medicine, law, business, and social sciences.

In addition to its main campus in Leuven, it has satellite campuses in Kortrijk, Antwerp, Ghent, Bruges, Ostend, Geel, Diepenbeek, Aalst, Sint-Katelijne-Waver, and in Belgium's capital Brussels. KU Leuven is the largest university in Belgium and the Low Countries. In 2017–18, more than 58,000 students were enrolled. Its primary language of instruction is Dutch, although several programs are taught in English, particularly graduate degrees.

KU Leuven consistently ranks among the top 100 universities in the world by major ranking tables. As of 2021, it ranks 45th in the Times Higher Education rankings, 84th according QS World University Rankings, 97th according to the Shanghai Academic Ranking of World Universities. For four consecutive years starting in 2016, Thomson Reuters ranked KU Leuven as Europe's most innovative university, with its researchers having filed more patents than any other university in Europe; its patents are also the most cited by external academics.

Although Catholic in heritage, KU Leuven operates independently from the Church. KU Leuven is open to students from different faiths or life-stances