The former Manchester City midfielder is already impressing for the Rossoneri.
"Coming here I have had a wonderful impression, even of the Milanello training complex," De Jong told Sky Sports 24.
"All players would like to train here and I'm happy to have joined AC Milan.
"It was not easy, I arrived on deadline day, but the group has welcomed me very well and I am very happy of that. AC Milan is in a transition season because some of the best players were sold and many veterans are gone.
"We have a young team that should be rebuilt but I am proud to be part of this project and I want to contribute to future successes.
"Watching the AC Milan of the 80s and early 90s I dreamed of playing here."