Ajax outclassed their opponents and cruised into a 2-0 lead at half-time through goals from Kolbeinn Sigthorsson and Christian Eriksen.
The second half was a procession and goals from Viktor Fischer, Siem De Jong and Danny Hoesen secured another domestic title for the Amsterdam club and condemned Willem II to relegation.
The result left the Ajax four points clear of nearest rivals PSV Eindhoven, whose 4-2 win over NEC Nijmegen was not enough to take the title race to the final round of matches.
It is the 32nd time Ajax have been crowned Dutch champions and coach de Boer becomes only the third man to lead the Amsterdam side to three Eredivisie crowns in a row after Rinus Michels and Louis van Gaal.
# Team MP W D L GF GA GD Pts 1 Ajax 33 21 10 2 81 31 +50 73 2 PSV 33 22 3 8 102 40 +62 69 3 Feyenoord 33 20 6 7 63 38 +25 66