Mikel San Jose opened the scoring after 13 minutes with a magnificent strike from the centre circle after an error from Barcelona keeper Marc-Andre Ter Stegen.
Barcelona’s inexperienced line-up then shipped three goals in a torrid second 45 minutes, leaving them with a huge task of overturning the deficit in the second leg on Monday at the Nou Camp.
It is the second time in two matches that Barcelona have conceded four goals, after their 5-4 victory over Sevilla in the Uefa Super Cup.
And the nature of this humiliating defeat will concern coach Luis Enrique as his side prepare to mount a defence of last season’s Treble – with their opening La Liga fixture coming against Athletic Bilbao at the San Mames.
Watch the Highlights and Goals Below