Ronaldo, a five-time winner of the Uefa Champions League, hit the woodwork with a first-half header in Jeddah and spurned a golden opportunity right at the death.
The Al Nassr captain, still to win a major trophy in his two and a half years in Saudi Arabia, left the pitch shaking his head.
Frontale, who were contesting their first Champions League semi-final, will now meet Jeddah-based Saudi Pro League club Al Ahli in Saturday’s showpiece.
They become only the fifth Japanese club to reach the final of the continent’s premier competition.
Frontale, who edged 2011 champions Al Sadd of Qatar 3-2 in extra-time on Sunday, began the semi-final with only one foreign player in their starting line-up, in stark contrast to big-spending Al Nassr’s seven.
It was Frontale who took a surprise lead on 10 minutes, when Tatsuya Ito met a looping clearance from Mohamed Simakan and sent a superb volley from the edge of the Al Nassr penalty area high into the top corner past Brazil goalkeeper Bento.
But just before the half-hour mark, Sadio Mane levelled.
The former Liverpool forward cut in from the left and then watched as his effort took a heavy deflection off Frontale stand-in captain Yuichi Maruyama, which was just enough to beat goalkeeper Louis Yamaguchi.
Soon after, Ronaldo attempted an audacious overhead kick but sent it well wide, while a minute later, he outjumped his marker to crash a close-range header against the upright.
However, four minutes before half-time, Frontale restored their lead.
Ito pounced on a loose ball and powered through Al Nassr’s flimsy defence only to see his effort blocked by Bento, but Yuto Ozeki finished the rebound expertly.
Al Nassr were all at sea at the back and in midfield, forcing manager Stefano Pioli to immediately make a double substitution.
Former Manchester City defender Aymeric Laporte replaced Ali Lajami, while Ali Al Hassan made way for winger Angelo, a summer signing from Chelsea.
In the second half, Bento got a touch to deny what seemed a certain third for Frontale but it mattered little when, with 14 minutes remaining, Laporte let Erison ghost past him on the Al Nassr byline before crossing for Akihiro Ienaga to volley home.
Substitute Ayman Yahya reduced the deficit in injury time to give Al Nassr hope, with Colombia international Jhon Duran wasting a brilliant chance moments later.
Yamaguchi denied Ronaldo twice in quick succession deep into injury time, before the Portugal captain then failed to make contact when through on goal, as the Japanese side held out for a famous victory. (AFP)
