Alajuelense came from 1-0 down in their second leg at home to Herediano, to win 3-1 on the night and 4-2 on aggregate to reach the Clausura final.
The rivalry between the two clubs has been a fierce one of late, with Alajuelense denying Herediano in four Apertura finals since 2010 before beating them in the semi-final stage on Wednesday to preserve their dominance in the Provincial Classic.
Dave Myrie had put Herediano ahead at the Estadio Alejandro Morera Soto in Alajuela, only for three quick-fire goals from the hosts to turn the tie on its head.
Just as they had done in the first leg, when Ariel Rodriguez equalised on 87 minutes away at Herediano, Alajuelense produced goals from Johan Venegas (84th minute), Alejandro Alpizar (89th) and Jose Ortiz (90th) to kill off the visitors.
Venegas' goal squared the tie, and it was headed for extra-time before Alpizar's intervention and Ortiz's sealer.
Deportivo Saprissa await Alajuelense for their 'Costa Rican Classic' final, after a 4-2 aggregate win over UCR.
Saprissa won their second leg at their Estadio Ricardo Saprissa Ayma home 2-0, although the tie was only killed off proper when Hansell Arauz doubled their advantage on the night on 89 minutes.
Carlos Saucedo had earlier put Saprissa in a strong position, with his 29th-minute opener resulting in UCR requiring two goals to advance.
The two-legged final kicks off on May 10, with the second leg a week later.
Alajuelense are vying to become the most successful club in Costa Rica's history, as they sit level with Saprissa on 29 trophies after the former won the 2013 Apertura title over Herediano in December.
