Diego Moreira was born on 6 August 2004 in Liège, Belgium, into a football family — his father, Almami Moreira, was a former Guinea-Bissau international who played for Standard de Liège. Diego came through Standard's youth academy before moving to Benfica in August 2020, where he signed his first professional contract and began developing through the club's reserve structure.
His most defining moment came in the 2021–22 UEFA Youth League, where he was a key contributor to Benfica U19's run to the title. He scored a brace and added two assists in a 4–0 quarter-final win over Sporting, then provided two more assists in the final — a 6–0 victory over Red Bull Salzburg — as Benfica claimed their first European youth title. He earned brief exposure to Benfica's senior side in May 2022 before joining Chelsea on a free transfer in July 2023.
After a loan spell at Lyon in the first half of the 2023–24 season, Moreira joined Ligue 1 side Strasbourg on a permanent transfer in August 2024, where he has taken on a more defensive role at left-back under manager Liam Rosenior.
A left-footed attacker with pace and an ability to beat defenders in one-on-one situations, Moreira holds dual Belgian-Portuguese nationality and represented Portugal at youth level before FIFA approved his switch to the senior Belgium national team in May 2025. He was subsequently called up to the Belgium squad for 2026 World Cup qualifying fixtures in June 2025.