Born on 4 January 2006 in Granollers, Catalonia, Marc Guiu joined Barcelona's La Masia academy in 2013 and worked his way through one of football's most demanding youth systems. He earned his first call-up to the senior squad in the summer of 2023 and wasted little time making an impression at the top level.
His breakthrough moment arrived on 22 October 2023, when he came off the bench in the 79th minute against Athletic Bilbao and scored the only goal of the game within 23 seconds of entering the pitch — his second touch of the ball. At 17 years and 291 days old, he became the youngest and fastest debutant to score for Barcelona in La Liga. He added a Champions League goal later that season against Royal Antwerp.
Guiu joined Chelsea on 1 July 2024 for a reported €6 million, signing a five-year deal. He contributed six goals in the UEFA Conference League during his debut season, including a first-half hat-trick against Shamrock Rovers, as Chelsea went on to win the 2024–25 competition. He also played a part in the club's 2025 FIFA Club World Cup triumph. After a brief loan spell at Sunderland was cut short, he returned to Chelsea and scored his first Champions League goal for the club in a 5–1 win over Ajax in October 2025.
Described as a classic centre-forward who is most effective inside the penalty area, Guiu relies on sharp movement, physical strength, and clinical finishing. Chelsea see him as a key part of their attacking options going forward. At international level, he was joint top scorer at the 2023 UEFA European Under-17 Championship with four goals.