Ronaldo headed home Joao Cancelo’s cross on the stroke of half time to add to his tally as the competition’s all-time top goalscorer.
However, David Neres curled home for the hosts at the beginning of the second period to leave the quarterfinal tie delicately poised at 1-1.
Ronaldo, who was playing for the first time since scoring a hat-trick against Atletico Madrid in last month’s round of 16, celebrated his goal in front of Ajax fans and appeared to have a beer cup thrown in his direction from the crowd.
The buildup to the hotly anticipated game had been overshadowed by crowd trouble in Amsterdam with Dutch police reporting that more than 140 people were arrested earlier in the day.
Most of those arrested were Italian, police said.
Ajax will feel unfortunate not to take a lead into the second leg next week after dominating for large spells of the game and creating a number of decent chances.
However, Douglas Costa also came close for Juventus, striking the post in the second period.