Two youths stole the empty tour bus when the driver left the vehicle unattended for a minute, reported Catalan daily, La Vanguardia.
They drove the distinctive red and blue tourist bus around the city for around half a kilometre before crashing into a parked car and fleeing the scene.
According to Barcelona's transport company, TMB, the incident happened at around 11am on the morning of Sunday, a peak tourist time in the city.
The bus driver, who was on his way to the start of his route on the plaza Catalunya, did not feel well, so pulled over and went into a bar to use its bathroom.
It was at this moment that two youths, suspected to be tourists, stole the bus and took it on a joy ride around the city for about half a kilometre before crashing into a parked car.
By the time local police reached the scene, the two youths had fled.
The tourist bus has been taken back to the depot where police will carry out tests on Monday in the hope of identifying the two bus thieves.
Barcelona has come under criticism for being too touristy with the new mayor Ada Colau promising to introduce controls in a bid to limit mass tourism.
The city has become a victim of its own success with many locals now complaining of "badly behaved" tourists ruing the city.