A Fresno police officer saw the couple physically fighting near the corner of Mariposa and E Streets around 10:00 pm.
The officer separated the two and saw that the woman was suffering from a stab wound to her upper body.
First responders rushed her to the hospital, and she is expected to recover.
Officers arrested the man suspected of stabbing her.
"Our male is in custody at this time. We're looking for some assault with a deadly weapon as well as some domestic violence charges," said Fresno Police Lt. Paul Cervantes.
Investigators say the suspect and victim were in a relationship. Domestic violence detectives are now looking into what led up to the stabbing.
If you are the victim of domestic abuse, or you know someone who is, there is help available 24/7. In Fresno County, call the Marjaree Mason Center at (559) 233-4357. In all other locations, call the National Domestic Violence Hotline at (800) 799-7233.