The Serie A club moved swiftly to bring the 31-year-old back to San Siro - reportedly on a free transfer - after he revealed he was finally ready to leave the Bernabeu, and Rossoneri vice-president Adriano Galliani flew to the Spanish capital on Saturday to thrash out a deal with the club and personal terms with the Ballon d'Or winner.
After arriving at Milano Linate airport by private jet on Monday morning, Kaka was mobbed by hundreds of fans who later assembled outside Milan's Via Turati headquarters to await confirmation a two-year contract had been signed, and a medical successfully passed.
That confirmation arrived in the form of mutual statements from Madrid and Milan, with the latter announcing on their official website "the permanent acquisition of Kaka's playing registration from Real Madrid."
The Brazil international then appeared on the Via Turati balcony to hold up the red and black striped shirt he last wore at San Siro in May 2009.
Earlier on Monday Kaka, who won the Champions League and Serie A title during his original six-year spell with Milan, had admitted he was delighted to be back.
He left to join Real in a reported 70million euros (£56million) move in 2009 but became an increasingly isolated figure during the three-year tenure of Jose Mourinho, while the signing of Wales winger Gareth Bale from Tottenham for a world-record 100million euros (£85.3m) would have pushed the Brazilian further down the pecking order.
Galliani and Kaka landed in Milan just before lunchtime and the Brazil striker told the Milan Channel on his arrival: "Four years have passed and now I'm back.
"It's been two weeks that I thought if I could come back, a return to Milan would be ideal for me.
"In the last few days, I've been dreaming of hearing the fans sing my name in the San Siro again."
After leaving Sao Paolo as a teenager in 2003, Kaka won the Champions League and one Scudetto during his six years in Milan - also landing the prestigious Ballon d'Or award in 2007 - and lifted the Primera Division trophy, the Copa del Rey and the Spanish Supercup with Real.
Kaka is confident he will team up well with Italy forward Mario Balotelli, who made the move to Milan from Manchester City earlier this year.
He added: "Mario is a great player and I want to do well with him. I hope I can do with him what I used to do with (Filippo) Inzaghi.
"I have a lot of desire to play and it's a very important year for me due to the World Cup, but right now I just want to do well for Milan, my team.
"I was very sad to leave this team and now it's great to be back. I'm very happy."
Galliani was delighted with the deal and believes Kaka will attract some long-lost fans back to the San Siro.
He told acmilan.com: "It was around 2.30 last night when the deal was done.
"Now I hope Kaka's orphans, the 12,000 or 15,000 fans that stopped coming to the stadium when he left, will come back now that Kaka's returned.
"He really made a huge effort to come back and has shown real affection for the jersey."