Boss De Boer suffers first Ajax defeat
Utrecht outclassed a lacklustre Ajax and were never in danger after Edouard Duplan scored twice in three minutes midway through the first half.
The French winger scored with a long-range effort before heading home his second from close range. Central defender Ismo Vorstermans added a third eight minutes from time.
Ajax, third with 38 points after 20 matches, dropped six points behind leaders PSV Eindhoven, who won 3-0 at VVV Venlo.
Second-placed Twente Enschede stayed one point behind the leaders with a 2-1 win over Groningen.
Wilfred Bouma opened the scoring for PSV at the near post after 14 minutes and Jeremain Lens volleyed the second on the stroke of half-time.
Atiba Hutchinson sealed PSV's victory immediately after the break.
Marc Janko struck twice as reigning champions Twente came from behind after Tim Matavz opened the scoring for Groningen in the 33rd minute.
The Austrian equalised from close range five minutes before the interval and notched the winner with nine minutes left when he scored from a narrow angle following a superb Theo Janssen pass.