A goal from 19-year-old Venezuelan forward Jan Hurtado 10 minutes from full-time set up a frantic finale but River held on to go through 2-1 on aggregate.
The "Millionaires" will play the winners of Wednesday's all-Brazilian clash between Flamengo and Gremio in the final in Santiago next month.
Urged on by 50,000 fans, Boca failed to get revenge in a repeat of last year's final, the second leg of which was played 10,000 kilometers away in Madrid -- and two weeks behind schedule -- after the original match was twice postponed.
The tone was set for a scrappy encounter when the kick-off was held up by 15 minutes to allow groundsmen using leaf-blowers to clear the pitch of thousands of paper sheets released like ticker tape by the fans.
When the match did eventually get underway, it was predictably breathless as Boca -- two goals down from the first leg -- desperately sought an early breakthrough.
But with Ramon Abila too often an isolated figure up front, Boca struggled for any fluidity, and River's center-halves Lucas Quarta and Javier Pinola cleared everything that came their way.
Boca's Toto Salvio swept the ball into the net after 20 minutes but celebrations were cut short when it was rightly disallowed for a clear handball without the ref needing a VAR check.
The ball struck defender Emmanuel Mas on the arm as he collided with teammate Augustin Almendra when flicking on Alexis Mac Allister's deep free kick.
River played the more composed football as the half wore on and the nearest Boca came to threatening Franco Armani's goal was a sliced clearance by Enzo Perez that drew a point-blank reflex save from the Argentina goalkeeper just before half-time.
Boca were better in the second half but still struggled for composure.
Former Manchester United and Juventus center-forward Carlos Tevez -- a sub in the first game -- played from the start here but failed to create enough chances.
The goal, when it came, was suitably scrappy. Mauro Zarate hoisted a free kick into the box, Lisandro Lopez won a header and with Armani stranded, Hurtado got the last touch as three Boca players scrambled for the loose ball.
It set up a frantic finish, as Boca peppered River's box with crosses. Substitute Sebastian Villa had a shot saved by Armani at his near post, and Mas rose highest to head a free-kick narrowly wide of Armani's right-hand post at the end of six minutes of injury time.
Brazilian referee Wilton Sampaio handed out eight yellow cards in a niggly match where chances were few and far between.
Flamengio play Gremio in the other semi-final at Rio de Janeiro's Maracana Stadium on Wednesday, with the match poised at 1-1.
It was a satisfying night for River Plate manager Marcelo Gallardo, who masterminded last year's 3-1 final victory over Boca after weeks of controversy.
Boca's players had refused to play the second-leg of last year's two-legged final at River's Monumental Stadium after their team bus came under attack from home fans.
Boca appealed to South America's football governing body CONMEBOL and the Court of Arbitration for Sport to have the trophy awarded to them, but both bids failed and they were beaten in Madrid.