Al Nassr were leading 1-0 through Sami Al-Naji's 13th-minute strike when Ronaldo took over with two late goals in the span of three minutes.
The Portuguese star's first came in the 77th minute. Following an Alex Telles cross that goalkeeper Paulo Vitor failed to deal with effectively, Ronaldo gathered the ball at the far post and fired a powerful shot into the net from a tight angle.
It was his next goal that truly had the fans in Riyadh on their feet. After Vítor raced from his box to stop an attack, the ball fell straight to Ronaldo who took a touch before sending an inch-perfect lob over goalkeeper and defenders that dropped right under the crossbar and into the net from more than 35 yards out.
The goal was Ronaldo's 15th in 13 league appearances this season.
With the win, Al Nassr moved up to second place in the Saudi Pro League, one point behind leaders Al Hilal who have a game in hand.