Pablo Hasel was given until last Friday to turn himself in, after being sentenced to nine months for glorifying terrorism, and slandering the crown and state institutions.
But he has tweeted that he is with supporters inside Lleida University.
"They'll have to break in to take me and jail me," he tweeted defiantly.
Besides attacks on the monarchy, the musician's tweets and lyrics accused police of torturing and killing demonstrators and migrants.
Lleida University is in the city of Lérida (Lleida in Catalan), 150km (90 miles) west of Barcelona. He is with about 20 supporters there.
More than 200 artists, including film director Pedro Almodóvar and Hollywood star Javier Bardem, have signed a petition against his jailing