The victim was in a residence in the 600 block of Birch Avenue when he was stabbed several times in his torso at about 4:30 a.m., police said.
The man's girlfriend told responding officers that they had fallen asleep in different rooms and that she had awaken to the sound of his screams, according to police. She did not report seeing a suspect.
The victim was in stable condition Monday at a local hospital.
Police said two teams of detectives are working on the investigation and preliminary reports indicate the victim was targeted.
Anyone with information regarding the incident is asked to call the Police Department's investigations services unit at (650) 522-7650. To remain anonymous, call (650) 522-7676.