El Larguero reports Ronaldo will take a €30m stake in the club and become their new president. The investment will clear Valladolid's €25m debts.
Current president Carlos Suarez will stay on, though as an executive president in charge of the day-to-day running of the club.
The operation will be formalised in the next two to three weeks.
It isn't the first time Ronaldo has invested in club football, having in the past been involved in the Fort Lauderdale Strikers in the US.