Eight-time Olympic gold medalist Bolt retired from athletics in 2017 and holds the world record for the 100 and 200 metres.
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However, Bolt believes that Ronaldo is the fastest of the two sporting icons.
"For sure Cristiano [is faster than me]," Bolt told Marca Sport Weekend. "For me he works out every day, he is a super athlete.
"He's always on top of his game, he works hard and he is focused. Right now I definitely think he would be faster than me."
At the age of 35, Ronaldo continues to break records after he became the second male player to score 100 international goals in September.
Ronaldo won his first international trophy with Portugal when they defeated France 1-0 in the Euro 2016 final, and also helped them to lift the inaugural UEFA Nations League title in 2019.
He is set to feature in the Nations League clash against France on Saturday.