The former Real Madrid forward, 33, appeared in court in the Spanish capital this morning.
The Portuguese international had already agreed a deal to pay the fine after pleading guilty last year to charges brought by Spanish state prosecutors and tax authorities.
He appeared relaxed as he arrived at court smiling with his fiancee Georgina Rodriguez, even signing an autograph on his way in.
Ronaldo spent around 45 minutes in court and, after formally accepting the deal offered by prosecutors, signed the agreement to pay the fine of nearly 19m euros (£16.7m)
The charges stem from his nine years at Real, whom he left for Italian club Juventus last summer.
Prosecutors accused him of four counts of tax fraud from 2011-14 worth 14.7m euros (£12.9) and using shell companies outside Spain to hide income made from image rights.