Ronaldo, who played for Cruzeiro as a teenager in the early 1990s before going on to become one of the most successful centre forwards in footballing history, did the deal with the help of Brazilian investment bank XP.
The transaction, which was done through Ronaldo's Tara Sports company and is still subject to what the bank called "a series of conditions," sees the 45-year-old invest 400 million reais ($70 million) in the Belo Horizonte club, which has spent two years in Brazil's second division.
"I am so happy to have concluded this operation," Ronaldo said in a video posted by the Cruzeiro president, adding, while holding one of the club's blue shirts, that he wants to "give back to Cruzeiro and take them where they deserve to be."
"We have a lot of hard to work to do. There's nothing to celebrate yet but we bring a lot of hard work and the ambition to make Cruzeiro great again."
On Sunday, he wrote on Instagram: "It's my turn to try and open doors for the team. Not as a hero. Not with superpowers to single-handedly change reality. But with immense responsibility. With intelligent and sustainable management for medium- and long-term growth."
"I don't have all the answers to the questions that I ask myself and I possibly won't have all the answers to the questions that you will ask me," he wrote in an open letter to fans.
"What I do know is that the kid who learnt at Cruzeiro that dreams can come true today makes me believe that it is possible to rescue the club from its crisis."
Neither he nor Cruzeiro provided any significant details but XP said in a statement "it seeks to help the Brazilian football industry with professionalisation, capitalization and opening new opportunities."
Ronaldo posted a blue heart and a fox -- the Cruzeiro mascot -- on Twitter, and a club spokesperson confirmed the deal.
Cruzeiro also told the fans they were "phenomenal", in a reference to Ronaldo's nickname as a player, "Ronaldo Fenomeno".
The deal comes a few months after the Brazilian Congress sanctioned a law allowing football clubs, historically fan-owned and closed off to outside investors, to become businesses.
"This is the first bit of business in a relevant new front for the investment banking market in Brazil, the country of football," said Jose Berenguer, CEO of Banco XP.
"I have no doubt this is transformational in the history of Brazilian sport. We will have clubs that are stronger, with the capacity for global investment. Brazilian football will never be the same again."