The Dutch defensive midfielder revealed that playing for the Italian giants has been a childhood dream of his, and is looking forward to the opportunity.
He also challenged his new team-mates to compete for honours both domestically and abroad - saying the quality of the squad is good enough to fight on all fronts.
"I'm happy, AC Milan is a great club and they are a new important chance for me in my life," De Jong told Sky Sports Italia.
"Everyone knows AC Milan's history. Everyone wants to play for AC Milan as a child.
"With this team we can compete both in Italy and in Europe, we have so many good players."
AC Milan CEO Adriano Galliani has praised his club's recent moves in the transfer market, and told coach Massimiliano Allegri that only winning Serie A would be an acceptable return in exchange.
"We think De Jong is one of the best defensive midfielders in Europe," he told TV 7 Gold.
"We believe we are making a good last market week with Niang, Bojan and De Jong. We thank president Silvio Berlusconi, who again has built a competitive AC Milan to win the Scudetto.
"(AC Milan coach, Massimiliano) Allegri has to worry now because our goal is no longer the third place, but the first. Among the players arrived in Italy from abroad, De Jong is the best."