Ronaldo finished as the tournament's top scorer with six goals in six games
The Portuguese forward, 38, played a key role as Al-Nassr came from behind to beat fellow Saudi side Al-Hilal 2-1.
Ronaldo cancelled out Michael's opener before Abdulelah Al-Amri and Nawaf Boushal were sent off for Al-Nassr.
But Ronaldo scored again in the first half of extra time to seal his first trophy since his lucrative move.
However, the five-time Ballon d'Or winner was carted off with five minutes left following a collision, just two days before his side's Saudi Pro League opener against Steven Gerrard's Al-Ettifaq.
The former Real Madrid, Manchester United and Juventus forward agreed a move to the Saudi Pro League in December and helped his side finish runners-up last season.
Several more big names have followed him to Saudi Arabia this summer, including new team-mates Sadio Mane, Seko Fofana and Marcelo Brozovic.
The trio were all denied by goalkeeper Mohammed Alowais in the first half at the King Fahd Stadium in Riyadh.
Former Barcelona and Zenit winger Malcom crossed for fellow Brazilian Michael to head in the opener on 51 minutes before Al-Amri was shown a straight red card on 71 minutes.
Although Ronaldo turned in a low cross by right-back Sultan Al-Ghannam to equalise on 74 minutes, Boushal also received a straight red card to put Al-Nassr down to nine men.
But eight minutes into extra time Ronaldo pounced on a rebound off the crossbar with keeper Alowais off his line to head home his sixth goal in six games in the tournament.
He finished as top scorer in the competition, which included the top clubs from Saudi Arabia, Qatar, the United Arab Emirates, Iraq, Morocco, Tunisia and Algeria.
-- Courtesy of BBC Sport