The Internacional striker scored two second-half goals to add to Romulu's first-half effort as the South Americans booked a final date with Mexico on Friday.
Brazil were forced to wait 38 minutes before they opened the scoring as South Korea enjoyed the better of the opening stages.
Hyunsung Kim nearly capitalised on a defensive mix-up to give them a surprise lead, but could only fire over the crossbar from inside the box.
It was shortly after that the pre-tournament favourites started to dominate possession, and that was converted into a one-goal lead when Romulu slotted under Lee Bum-Young following build-up work from Premier League pair Sandro and Oscar.
Reported Tottenham Hotspur target Leandro was the star of the second period as he netted twice in eight minutes to establish Brazil's advantage.
He firstly powered home from Neymar's accurate cut-back before bundling the ball over the line from close range following a goalmouth scramble.
There was no sign of a remarkable South Korea comeback as Brazil coasted through the final 20 minutes.
They will face Mexico at Wembley, while Japan and South Korea will meet before Friday's showpiece to decide who takes home the bronze medal.