The sibling breakout occurred at the Modelo de Barcelona prison.
In the incident, a prisoner who was doing time for domestic violence, swapped places with a brother who had come to visit him.
The escape took place at the end of a family visit for the prisoner, sources close to the case told Spanish news agency EFE. When the family catch-up ended, the Pakistani prisoner swapped places with his brother and managed to walk out of the jail.
Apparently the two brothers are very similar in appearance.
Once the security breach had been spotted, prison guards quickly detained the 'innocent' brother who had remained behind bars. He is now awaiting his day in court.
The escaped brother remains, meanwhile, on the loose.
A representative of the prison workers' union in Catalonia CSIF, Juan Luis Escudero Pérez, blamed the incident on a lack of technology at the prison where the breakout occurred.
He said without electronic fingerprinting facilities, all fingerprint checks were done visually.
The Catalonian government is now looking at introducing this new technology.