The 42-year-old, who played for Spanish giants Real Madrid and Barcelona during an 18-year career, said his immediate aim was consolidating the club’s top-flight status.
Valladolid were promoted this season and have two points from their opening three games.
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"We want Real Valladolid to consolidate in the top flight and keep building hopes from there," said Ronaldo.
"It will not stop there because we want to grow to where our hopes allow us to."
Carlos Suarez, who Ronaldo purchased his stake from, will continue as club president.
"The arrival of Ronaldo puts Valladolid on the map and will allow the club to take a leap forward in quality. I welcome him to his home," said Suarez.
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