The Brazilian forward moved from Inter Milan to Real for 45 million euros ($56 million) in 2002, signing a four-year deal with an option for one further season.
Real announced on Tuesday that the two parties had agreed to prolong the deal for two further years.
Ronaldo, 27, is the leading scorer in Spain's Primera Liga this season with 22 goals.
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- Real Madrid agree deal to sign Ronaldo
- Ronaldo denies Chelsea talk
- Ronaldo confident of keeping 35-goal promise
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