Košarkaški klub Zagreb (English: Zagreb Basketball Club) is a professional basketball club based in Zagreb, Croatia. It competes in the Croatian League, the Adriatic League and in the 2011/12 season for the first time in the modern Euroleague. For sponsorship reasons since 2007 the club is known as Zagreb Croatia Osiguranje.
The club was founded in 1970 under the name KK Siget after Siget neighbourhood in Zagreb. In 1976 club was renamed to KK Novi Zagreb to represent Novi Zagreb ("New Zagreb"), part of Zagreb located south of the Sava river. After Croatia became independent country, "Novi" was just dropped out and the club name got its simpler and present form - KK Zagreb. From the beginnings Zagreb players were known under the nickname Ants as a symbol of hard work and team spirit.
Since its establishment KK Zagreb worked its way up over 19 years to join the famed first division in ex-Yugoslavia in 1989. Though it carries the name of a great basketball capital KK Zagreb had always lived a bit in the shadow of cross-town rival Cibona but since the beginning of the Croatian League in the early 1990s Zagreb has made a name of its own, becoming one of the better teams in the country. Until late 2000s club was mainly known for its youth categories which won numerous Croatian and European titles, while senior team was always in the shadow of Croatian basketball "Big Three" - Cibona, Zadar and Split. In recent years, when its became a permanent member of the Eurochallenge Cup competition, the club increased its budget with an intention to make another step forward in its ambitious plans and won its first domestic trophies, the Croatian Cup in 2008 and 2010. It was only a matter of time, when the team will move from its traditional base in school sports hall Trnsko (capacity 3,000) to a bigger one, which happened in 2010, when they moved to Dražen Petrović Basketball Hall, home of their better known rival Cibona. Moving to a new and bigger hall proved to be some kind of "growing up" for KK Zagreb as the club won double title, winning both Championships of Croatia and Croatian Cup in 2011.