Gremio coach Renato Gaucho claimed in the build-up that he was a better player than Ronaldo, and the Portugal star provided the perfect response on the pitch, frequently troubling the Brazilian side and scoring the all-important goal.
Madrid often looked susceptible on the counter-attack in their 2-1 semi-final win over Al Jazira, though the same could not be said against Gremio as the European champions exerted complete control in the first half.
Plenty of chances fell Madrid's way, though it took them until the second half to break the deadlock and there was more than a hint of fortune about it, with Ronaldo's free-kick somehow finding its way past Marcelo Grohe and making him the first player to score in successive Club World Cup finals.
Luka Modric saw a 25-yard effort of his own tipped on to the post soon after, as Madrid's search for a second left them frustrated.
Ultimately it did not matter, as Gremio failed to cause the LaLiga team any problems in spite of their own desperation and Zinedine Zidane's men had little difficulty holding on to claim the Club World Cup title for a third time in four years.
It did not take Madrid long to establish dominance and they kept the South American champions penned in their defensive third for extended periods during the early exchanges.
The Spanish side were not shy in getting shots away, though most of them were from distance and caused Gremio goalkeeper Grohe few problems.
Grohe had to be quick off his line in the 21st minute, however, as he rushed out to smother Raphael Varane's effort after Sergio Ramos flicked on a corner, with Modric shooting wide from 20 yards a few moments later.
Gremio eventually had a chance of their own and they went close, Edilson's 40-yard free-kick flying agonisingly over.
And Ronaldo threatened in similar fashion just before the break when his long-range shot from a set-piece grazed the roof of the goal.
Madrid's dominance showed no sign of letting up after the interval and few could claim they were not deserving of the lead when they finally ended Gremio's resistance.
Ronaldo dazzled Jailson with some quick stepovers and lured him into a foul, before shooting through the wall from the resultant 25-yard free-kick and finding the bottom-left corner.
Another final in 2017, another goal for @Cristiano!
Champions League final
Spanish Super Cup
Club World Cup final#RMCWC | #HalaMadrid pic.twitter.com/tIFmjDmsuF — Real Madrid C.F. (@realmadriden) December 16, 2017
The Portugal star then had another ruled out a few minutes later due to Karim Bernzema straying offside.
Modric nearly added a legitimate second just after the hour mark, but Grohe managed to tip his effort on to the post.
Gremio's need to throw men forward provided Madrid with several opportunities to counter during the final stages and, although they failed to add to the score, Zidane's men comfortably held on to lift the Club World Cup, giving them the perfect boost ahead of El Clasico next Saturday.
Key Opta facts:
- Real Madrid have won five titles in the same calendar year for the first time ever (La Liga, Champions League, Supercopa de Espana, UEFA Super Cup, Club World Cup).
- Cristiano Ronaldo is the all-time highest-scoring player in the Club World Cup (seven goals).
- Madrid have become the first club to be world champions in consecutive years since Sao Paulo under Tele Santana in 1992 and 1993 (Intercontinental Cup).
- Madrid have matched Barcelona (three) as the side with the most Club World Cup wins.