The forward grabbed his crotch to mimic Atletico manager Diego Simeone during his team's 3-0 second-leg victory in Turin, which saw them overturn a 2-0 deficit. Matias Grez of CNN reported on Thursday that Ronaldo has received a €20,000 (£17,000) fine from the governing body on Thursday.
Simeone was fined the same amount for his celebration in the first leg of the tie at the Wanda Metropolitano stadium but avoided a touchline ban, per BBC Sport.
The news will be a big boost to Juventus and their hopes of lifting the Champions League for the first time since 1996.
Ronaldo was brought to Turin in summer 2018 for €100 million (£88.3 million) with the aim of delivering success in Europe.
The Portuguese superstar has won the continent's top competition five times in his career. His first title came with Manchester United in 2009, and he went on to win it four times with Real Madrid, including the last three consecutively.
Ronaldo was in inspirational form in the last 16 after defeat in the first leg had Juventus heading for an early exit from the competition, netting a hat-trick to seal a 3-2 aggregate win and a place in the quarter-finals.
Ronaldo suggested to Sky Sport Italia (h/t Ben Gladwell at ESPN FC) after the win that his goals were the reason Juventus signed him.
"This had to be a great night and it was one, not just for my goals but for the team. Maybe this is why Juve signed me. I just do my job and it was a magic night. We've not done anything yet, but we're very proud."
Ajax have already pulled off one of this season's biggest Champions League shocks, defeating Ronaldo's former club and the competition's holders Real Madrid in the last 16 to book their place in the quarter-finals.