The 37-year-old joined the Saudi Arabian side on a deal that runs until 2025.
The Portugal captain is a free agent after leaving Manchester United following a controversial interview in which he criticised the club.
Ronaldo says he is “eager to experience a new football league in a different country”, according to BBC Sport.
Al Nassr – nine-time Saudi Pro League champions – described the signing as “history in the making”.
The club said it would “inspire our league, nation and future generations, boys and girls to be the best version of themselves”.
In the summer, Ronaldo turned down a £305m deal to join another Saudi team – Al Hilal – because he was happy at United.
Al Nassr hopes to sign Sergio Busquets from Barcelona when his contract expires by June.