Bolt retired from athletics in 2017 but the eight-time Olympic gold medallist still holds the world records over 100 and 200 metres.
The 34-year-old clocked a world-record time of 9.58 seconds in the 100m at the 2009 World Championships in Berlin, while he also ran 200m in 19.19s 11 years ago in the German city.
But Bolt feels five-time Ballon d'Or winner and footballer Ronaldo is faster.
"For sure Cristiano [is faster than me]," Manchester United fan Bolt, who also tried to forge a career in football post-athletics following trials with the likes of Borussia Dortmund and A-League side Central Coast Mariners, told Marca Sport Weekend.
"For me he works out every day, he is a super athlete.
"He always on top of his game, he works hard and he is focussed. Right now I definitely think he would be faster than me."
Ronaldo has scored six goals in four Serie A appearances for Juventus this season, while the 35-year-old netted in Portugal's 7-0 friendly rout of Andorra on Wednesday.
After bursting onto the scene at Sporting CP, Ronaldo spent six years at United, where he won three Premier League titles and the Champions League among other honours.
Ronaldo left United for Real Madrid in 2009, having scored 118 goals for the Red Devils across all competitions.
In the Spanish capital, Ronaldo became Madrid's all-time leading goalscorer with 450 – 311 of those coming in LaLiga.
Ronaldo claimed four Champions League crowns, a pair of LaLiga trophies, three Club World Cup honours and two UEFA Super Cups among other silverware before joining Juventus in 2018.
In Turin, Ronaldo already has 71 goals and two Serie A trophies to his name amid speculation he could depart for Paris Saint-Germain or United.