The 26-year-old man identified as Alejandro G. was walking with friends in the early hours of Saturday in Kreuzberg, one of Berlin's most popular districts for nightlife.
As Alejandro approached a bus stop, police say an unknown man suddenly attacked and stabbed him with a knife.
He and his friends started to run away, but police told The Local that the attack happened so quickly, the friends did not realize Alejandro had been stabbed until he collapsed to the ground.
"This attack is hard to understand," police spokesman Jens Berger told The Local. "They did not know each other, they did not have contact before, no fight. It was sudden with no known justification."
Emergency responders arrived quickly to the site, but Alejandro died later at a hospital.
Alejandro had come to Berlin from Zaragoza about a year ago to study German and work, Berger said.
Police have been looking for more witnesses in the area, passing out flyers and talking to people nearby to see if they saw anything that night.
The suspect is described as a slim man between the ages of 25 and 30 who looks "southern European" and with either a moustache or a goatee.
"This is an exceptional case," Berger said. "It seems completely without reason, completely without motive and is even unusual for the investigators."