Ronald Koeman (Dutch pronunciation: [ˈroːnɑlt ˈkumɑn] ^ⓘ; born 21 March 1963) is a Dutch professional football manager and former player who is the manager of the Netherlands national team. Koeman scored over 250 goals whilst playing in defence for the majority of his career. Koeman was capable of playing both as a defender and as a midfielder; he frequently played as a sweeper, although he was equally known for his goalscoring, long-range shooting, and accuracy from free kicks and penalties. Because of his goalscoring, he is considered one of the best attacking center backs of all time.

Born in Zaandam, Koeman began his career at Groningen before transferring to the Netherlands' most successful club Ajax in 1983, where they won the national Eredivisie title in 1984–85. He then joined Ajax's rivals PSV in 1986, winning three consecutive Eredivisie titles (1986–87, 1987–88 and 1988–89) and the European Cup in 1988. Ronald Koeman is one of five European players to ever win a treble with their club and a cup with their national team in the same year. The other four players are his teammates Hans van Breukelen, Berry van Aerle, Gerald Vanenburg and Wim Kieft. In 1989, Koeman moved to Barcelona and became part of Johan Cruyff's "Dream Team", helping the club win La Liga four consecutive seasons (1991 to 1994), and the 1991–92 European Cup, where he scored the winning goal of the final against Sampdoria.

At international level, Koeman was one of the stars of the Netherlands national team, alongside Marco van Basten, Ruud Gullit, Frank Rijkaard and Dennis Bergkamp. During his career with the Netherlands, Koeman won UEFA Euro 1988 and played at UEFA Euro 1992, as well as the 1990 and 1994 FIFA World Cups, captaining the team at the latter.

In his managerial career, Koeman has won three Eredivisie titles: twice with Ajax (2001–02 and 2003–04) and once with PSV (2006–07). He is the only individual to have both played for and managed the "Big Three" of Dutch football: Ajax, PSV and Feyenoord. Abroad, he had spells in Portugal with Benfica and Spain with Valencia, coaching Los Ché to victory in the 2007–08 Copa del Rey, and managed Premier League clubs Southampton and Everton in the 2010s. He was the manager of the Netherlands national team between 2018 and 2020, finishing runners-up in the 2019 UEFA Nations League. In August 2020, he was appointed manager of Barcelona, with whom he won the 2020–21 Copa del Rey, but was sacked in October 2021, following a poor start to the new season. In 2023, he was reappointed as the manager of the Netherlands national team.

Koeman started his professional career at Groningen, making his debut at the age of 17 years and 183 days in a 2–0 win over NEC in the Eredivisie. This made him the third-youngest player in the club's history, after Piet Wildschut and Bert de Voogt. Thirty-three goals from ninety appearances in his three seasons at the club saw the young defender called up by the Netherlands national team and earn a transfer to Eredivisie champions Ajax. After failing to defend their title in Koeman's first season at the club, the Amsterdam team regained the championship in 1984–85. The following season saw Johan Cruyff take over as Ajax head coach and, despite scoring 120 goals in 34 Eredivisie matches and winning the KNVB Cup, de Godenzonen could only finish second in the league behind rivals PSV.

In the summer of 1986, Koeman controversially transferred to PSV to play for Hans Kraay's champions. Towards the end of the 1986–87 season, Kraay resigned and was replaced by Guus Hiddink, under the management of whom PSV overtook league leaders Ajax in the final weeks of the season to defend their league title. Koeman enjoyed further success with Hiddink and PSV in the following seasons, as the team also won the 1987–88 and 1988–89 Eredivisie titles and the club's first, and to date only, European Cup against Benfica in Stuttgart on 25 May 1988. PSV had also won the KNVB Cup in both 1988 and 1989, making their successes in the two years trebles and doubles respectively. In his three seasons at PSV, Koeman scored 51 goals in 98 league appearances, averaging more than one goal every two matches. During 1987–88 season, he recorded the highest scoring season of his club career, with 21 goals scored in the league.

In 1989, Koeman re-joined his former Ajax coach Johan Cruyff at Barcelona, where he became a member of the famous "Dream Team". During his first season at the club, Barcelona won the Copa del Rey, beating Real Madrid 2–0 in the final. Along with players such as Hristo Stoichkov, Romário, Pep Guardiola and Michael Laudrup, Koeman helped the club win La Liga four consecutive seasons years (1991 to 1994). He scored the only goal of the 1992 European Cup Final with a free-kick from the edge of the 18-yard box against Sampdoria at Wembley to make Barça European champions for the first time in their history. With this, he became the first player to score in two consecutive finals of different European competitions, having scored Barcelona's consolation goal in a 1–2 defeat against Manchester United in the 1991 European Cup Winners' Cup Final.

Koeman was known for his powerful right-footed free kicks and deadball ability where he scored many vital goals for the team. One of his best strikes in La Liga came in the memorable 5–0 win over Real Madrid in 1994, with his bending free kick making the scoreline 2–0. Koeman was joint-top scorer with eight goals in the 1993–94 UEFA Champions League, in which Barcelona were beaten 0–4 in the final by A.C. Milan at the Olympic Stadium in Athens.