For long periods of this game in Abu Dhabi, it looked like Brazilian side Gremio would frustrate Real Madrid and perhaps even pinch a victory as the Spanish giants became increasingly frustrated in their attempts to break down a disciplined defence.
Real dominated possession in the first half but only one of their 10 chances before the break was on target.
They switched to attempting to find joy from long distance but efforts by Luka Modric, Ronaldo and Karim Benzema were all wayward.
However, eight minutes into the second half Real were awarded a free-kick 25 yards from goal when Ronaldo was tripped and the Portugal international picked himself up to curl the ball through a gap in the wall and into the net.
It was Ronaldo's fourth goal in three Club World Cup finals and extended his record of scoring in significant finals for Real Madrid, having also hit three goals in three Champions League finals and two goals in two Copa del Rey finals.