Jose Miguel Arenas Beltran -- better known as Valtonyc -- was sentenced last year for lyrics that Spain's National Court felt warranted prison, and the Supreme Court upheld this in February.
The reference to the Basque Country hinted at violence by ETA, the separatist group that for decades staged scores of attacks across Spain that left hundreds of officials and civilians dead.
The 24-year-old also sang against Jorge Campos, the former president of the Balearic Circle, a foundation that fought against the imposition of the Catalan language in the Balearic Islands, where Valtonyc comes from.
Lyrics included: "Jorge Campos deserves a nuclear destruction bomb" or "I will pull out his artery and everything else necessary."
Valtonyc had been freed on bail until his prison entry deadline, due Thursday.
But on Wednesday, he posted a cryptic tweet: "Tomorrow they will knock down the door of my house to put me in jail. For some songs. Tomorrow Spain is going to make a fool of itself, once more.
Mañana es el día. Mañana van a tumbar la puerta de mi casa para meterme en la carcel. Por unas canciones. Mañana España va a hacer el ridículo, una vez más. No se lo voy a poner tan fácil, desobedecer es legítimo y obligación ante este estado fascista. Aquí no se rinde nadie.— Josep V. 🎗️ (@valtonyc) May 23, 2018
"I'm not going to make it easy for them, it's legitimate and an obligation to disobey this fascist state. Here, no one surrenders."
Several local media in the Balearic Islands and neighbouring Catalonia said he had fled Spain, with reports that he was meeting lawyers in Brussels, but there was no official confirmation.
Concerns have mounted over the past few months as several Twitter users and rappers have been judged -- and some of them jailed -- for glorifying terrorism or insulting the king for comments or lyrics.
In March, Amnesty International said a Spanish law banning "glorification of terrorism" had created a "chilling" environment in which people are increasingly afraid to express alternative views or make controversial jokes.
But other legal experts stress free speech has its limits.
It's "not permission to say whatever one wants," said Antonio Torres del Moral, constitutional law specialist.