Siem de Jong netted a rebound after eight minutes to put De Boer's team ahead and the striker added a second 13 minutes into the second half to start the celebrations.
"For the fans, last season was special because they waited so long for the third star on the shirt," De Boer, who took over from Martin Jol in December 2010, told reporters.
"But for me and my staff this one feels more beautiful seeing what happened this season.
"All the injuries we had and then the turbulence through all the problems in our board, but we stayed loyal to our vision," added the coach who also won five titles with Ajax as a player.
"I know Frank well, and he did a great job and won a deserved title," he said.
"Nobody gave a dime for the chances of Ajax when we beat them at the end of January, but Frank stayed calm despite all the board turbulences."
"He never used it as an excuse and kept everything far away from his players and that deserves a big compliment," added Koeman.
Otman Bakkal scored from a rebound after a Ruben Schaken effort which hit the post and three goals within 11 minutes from Jerson Cabral, Sekou Cisse and Schaken sealed Feyenoord's win.
