A 75-year-old was stabbed to death in her apartment in the Berlin district of Prenzlauer Berg.
A 20-year-old neighbor was arrested as a suspect, the police said on Saturday.
A homicide squad is investigating.
According to previous knowledge, the young man is said to have broken into the neighbour's apartment in Oderbruchstrasse by force on Friday evening around 8:10 p.m.
There he is said to have stabbed the elderly woman with a knife.
An emergency doctor could only determine her death, said a fire department spokesman.
According to the police, the suspect initially fled.
A little later he returned to the scene and turned himself in to the police.
Witnesses had therefore alerted the emergency services.
The motive for the crime is still unclear, no information has yet been given.
The police union wrote on Twitter that the man's arrest was made possible by the courageous behavior of two media representatives.