The former Holland midfielder played for the club for 10 years before moving to Brazilian club Botofogo in 2012.
He announced on Tuesday that he was calling time on his playing career to succeed Massimiliano Allegri as Rossoneri coach and he arrived on Wednesday at Milan's Linate airport, where he was greeted by a crowd of journalists and photographers.
Seedorf told the Milan Channel: " You cannot say that I'm back, because I feel like I've never been away. It's great for me to see all this enthusiasm.
"We have much to do to bring AC Milan back to the top. I'm very happy.
"Two years ago, Mr Berlusconi had planned everything. Right now I feel great emotion and sense of duty.
"I have to give 100 per cent to rebuild enthusiasm and give security to the team. I left Milan in May of 2012 at the end of a cycle. My job will be to reopen another."
Seedorf returns with Milan remarkably languishing in the bottom half of the Serie A table, lying 10 points outside the European places and 30 adrift of leaders Juventus in 11th place following Sunday's shock 4-3 defeat to Sassuolo.