Joško Gvardiol was born on 23 January 2002 in Zagreb, Croatia. He began playing football at the age of seven at NK Trešnjevka before joining the academy of NK Dinamo Zagreb, where he was converted from a central midfielder into a centre-back by coach Dalibor Poldrugač. He made his senior debut for Dinamo in the Croatian top flight in October 2019 and scored in only his second league appearance, becoming one of the youngest scorers in the league's history at the time.
In September 2020, RB Leipzig agreed a deal worth €16 million plus add-ons to sign Gvardiol on a five-year contract from the following summer. He went on to establish himself as one of the more reliable defenders in the Bundesliga during his two seasons in Germany.
His profile rose sharply at the 2022 FIFA World Cup in Qatar, where Croatia finished third. Gvardiol scored in the third-place playoff victory over Morocco, and his performances throughout the tournament drew widespread attention from top clubs. He has made over 27 senior international appearances for Croatia since his debut in 2021.
In August 2023, Manchester City signed Gvardiol for a reported fee of around £77.5 million, making him the second most expensive defender in transfer history at the time, behind Harry Maguire. He was assigned the squad number 24. A left-footed, ball-playing centre-back comfortable stepping into midfield, Gvardiol joined City as they successfully defended their Premier League title for a record fourth consecutive season in 2023–24.