The club did not disclose the transfer fee, but local media reported it was 60 million euros.
Otavio scored 31 goals in 283 games in all competitions for Porto after joining the team in 2014.
"We have officially signed the Portuguese Star Otavio. Huge Welcome to The Best Player in Liga Portugal," Al Nassr posted on messaging platform X, previously known as Twitter.
Otavio joins an Al Nassr squad that also includes Cristiano Ronaldo, Sadio Mane, Marcelo Brozovic, Alex Telles and Seko Fofana.
Luis Castro's team have made a poor start to the Pro League season, losing both their opening matches.
The Saudi Public Investment Fund (PIF) has announced a Sports Clubs Investment and Privatisation Project involving league champions Al Ittihad, Al Ahli, Al Nassr and Al Hilal.
PIF owns 75% of each of the four clubs, while their respective non-profit foundations own 25% of each.
