The Portugal international, a five-time Ballon D'Or winner, converted from the spot three minutes into added time, having failed to score on his debut last month.
Ronaldo signed a two-and-a-half year deal with Al-Nassr in December, reported to be worth over 200 million euros, and was appointed captain shortly after his arrival.
Al-Nassr now top the standings after 15 games, level on 34 points with second-placed Al-Shabab but with a match in hand. They next travel to Al-Wedha on Thursday.