Koeman started his professional career at FC 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 Eindhoven 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, he recorded the highest scoring season of his club career, with 21 goals scored in the league.
Koeman started his professional career at FC 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 Eindhoven 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, he recorded the highest scoring season of his club career, with 21 goals scored in the league.
Ronald Koeman