Marcelo Alberto Bielsa Caldera ([maɾˈselo alˈβeɾto ˈβjelsa], nicknamed El Loco Bielsa [ˈloko ˈβjelsa], meaning 'The Madman Bielsa'; born 21 July 1955) is an Argentine professional football manager who is the current manager of the Uruguay national team. He is widely regarded as one of the most influential coaches of all time. Bielsa played as a defender for Newell's Old Boys, Instituto, and Argentino de Rosario.

Bielsa played as a defender in Newell's Old Boys' First Division team and was a member of Argentina U23 in the 1976 Pre-Olympic Tournament, but retired when he was 25 to focus on coaching. Bielsa has managed several football clubs and also the national teams of Argentina and Chile. He developed his career as coach of Newell's Old Boys where he won several titles in the early 1990s, before moving to Mexico in 1992, briefly coaching Club Atlas and Club América. Bielsa returned to Argentina in 1997 to manage Vélez Sarsfield, leading them to the 1998 league title (Clausura).

His personality and gestures during his stint in Chile captured the attention of media and unleashed a series of minor controversies both in sports and politics. He had a two year-spell in Spain at Athletic Bilbao between 2011 and 2013, leading them to domestic and continental cup finals in the first season, though they lost both. In May 2014, Bielsa was appointed coach of Marseille, starting with good results but finishing outside the expected first three places in Ligue 1, resigning after just over a year at the French club. He remains highly regarded by OM fans for his offensive playing style and personality.

In June 2018, Bielsa was appointed manager of then-Championship club Leeds United, leading the club to promotion back to the Premier League in 2020 after a 16-year absence as a result of winning the 2019–20 EFL Championship. Leeds United is the club at which he spent the most time as manager in his career, taking charge of 170 games before departing in February 2022. He is considered a cult figure among Leeds fans.

In 2023, he became manager of Uruguay, becoming only the second non-Uruguayan manager to coach the team. He led Uruguay to two record-breaking wins against Brazil and Argentina before subsequently achieving qualification for the 2026 FIFA World Cup.

In 1980, after retiring from playing in football, Bielsa decided to start a career as a football manager. His first job was coaching the youth divisions of Newell's. In 1990, he was given the task of managing their first team, where he would go on to win the 1990 Torneo Apertura and the 1990–91 Torneo Integración, defeating Boca Juniors on penalties. Bielsa managed the squad that competed in the final of the 1992 Copa Libertadores, losing to São Paulo on penalties. Weeks after the Libertadores defeat, Bielsa and Newell's won the 1992 Torneo Clausura. After a period in Mexico, he returned to Argentina and won another league title – the 1998 Clausura – while coaching Vélez Sarsfield.

In 1998, Bielsa was given the job of manager at Spanish La Liga side Espanyol, but he soon left after being offered the management of the Argentina national team later that year, taking over after a four-year period by Daniel Passarella as manager. At the 1999 Copa América, Argentina were beaten 3–0 by Colombia in a game where Argentina forward Martin Palermo missed three penalties and Bielsa was sent off. On Bielsa's post match reaction, Tim Vickery of the BBC wrote:

In the press conference he sat staring into space, refusing to make eye contact with anyone – his usual stance. He was asked what he had made of the referee's performance. If the question was predictable, the answer was anything but. "One doesn't usually have the habit of commenting on referees, but…" he said, leaving everyone to believe he would continue with a rant about a joker running amok with a whistle. Instead he proceeded with, "but in respect of my expulsion, the referee was absolutely correct because I protested in an ill mannered form."