A Mikel Oyarzabal first-half penalty was enough to make Real Sociedad the first club to book their place in the final of the Spanish Copa del Rey, for the first time in 32 years.
There was little to choose between Sociedad and its hosts Mirandes, with the two teams blocking any play in the midfield.
Oyarzbal, however, converted his spot-kick just minutes before halftime, after a Mikhael Malsa handball.
Although Mirandes hasn't made it to the finals, their cup run is commendable, as they knocked out top-flight sides Celta Vigo, Sevilla, and Villarreal, to get to the semis.
Sociedad beat Mirandes 3-1 on aggregate and they will face Granada or Athletic Club Bilbao in the final of the Spanish competition.
The match will be played tonight at Granada's Los Carmines Stadium, with Bilbao defending a 1-0 lead.