Ronaldo, who played for FC Barcelona, Inter Milan and Real Madrid in his glittering career, bought 51 per cent of the shares in the club from Valladolid club president Carlos Suarez at a price of around 30 million euros, reports Xinhua news agency.
Valladolid, who have traditionally been a yo-yo-club in Spain, returned to the Spanish top-flight this summer for the first time since suffering relegation in 2014. At a press conference to confirm the purchase, Ronaldo said his ambition was to ensure the club stays in the Spanish first division.
"We want Real Valladolid to consolidate in the top flight and keep building hopes from there," said the former striker, who added that he had more ambitions than merely avoiding the drop.
"It will not stop there because we want to grow to where our ambitions allow us to," he commented.
Suarez will remain in his role as president and said Ronaldo's arrival would put the club "on the map and allow it to take a stride forward in quality".
Valladolid managed two draws as well as a home defeat to Barcelona in their three matches so far this season.