Kortabarria was born in Mondragón, Gipuzkoa. During his career he played solely for Real Sociedad, making his first-team – and La Liga – debut on 19 September 1971 against Deportivo de La Coruña and finishing his first season with 14 appearances.
From there onwards, Kortabarria rarely missed a game, contributing 30 and 31 matches respectively as the Basques won back-to-back leagues (whilst adding five and six goals, his career-bests) with him as captain. In the last round of the 1980–81 campaign, he opened the scoring through an early penalty as Real drew 2–2 at Sporting de Gijón to eventually edge Real Madrid on goal difference.
After losing his place to younger Agustín Gajate, Alberto Górriz and Juan Antonio Larrañaga, all brought up in the club’s youth academy, Kortabarria chose to retire at the end of 1984–85 aged 35. He made 442 appearances in all competitions, eighth all-time for Real Sociedad.