In his first spell at Ajax from 1988 to 1991, he won the Eredivisie title in 1989–90 under manager Leo Beenhakker. After two seasons at Twente from 1991 to 1993, he returned to Ajax under manager Louis van Gaal, and had his most successful spell as a player, winning three consecutive Eredivisie titles in 1993–94, 1994–95 and 1995–96. He also won the 1995 UEFA Champions League, the 1995 Intercontinental Cup, and the 1995 UEFA Super Cup. He was also a runner-up of the 1996 UEFA Champions League, where Ajax lost on penalties to Juventus. When Louis van Gaal left Ajax for Barcelona in July 1997, Morten Olsen became the new Ajax manager, and De Boer won a fifth Eredivisie title in 1997–98 and also won the 1997–98 KNVB Cup.