Ronaldo's low strike, following a back-heel from team-mate Miralem Pjanic, brought Juventus level at the San Siro.
Radja Nainggolan had given the hosts the lead with a spectacular 25-yard volley after Matteo Politano's flick.
Ronaldo has now scored 27 goals for Juventus after five for Sporting Lisbon, 118 at Manchester United and 450 for Real Madrid.
His 600th career club goal came on the same day as Lionel Messi scored his 598th Barcelona goal, during their match against Levante.