Emergency services were called to an address in Battersea, south London, at 12.38pm.
They found a woman, believed to be in her 30s, suffering a stab injury.
Paramedics from London Ambulance Service (LAS) worked for an hour but she was pronounced dead at 1.41pm.
Her next of kin have not yet been informed.
A man, 40, was arrested at the scene on suspicion of murder.
Vickie Foot, 43, who lives nearby, said the incident was “absolutely shocking” and that she saw the arrest.
She added: “They handcuffed him and put him into the van. He looked quite calm.”
Police told residents: “It’s not gang-related.”
One man who lives on the road told Sky News police had cordoned off the area and told residents to stay inside their homes.
Forensics officers in plastic overalls have been seen outside the house.
An LAS spokesman said: “We sent an incident response officer, an advanced paramedic, a single responder in a car and an ambulance crew. We also dispatched London’s air ambulance.
“Sadly, despite the efforts of our medics, a person died at the scene.”
Metropolitan Police said a post-mortem will be carried out in due course.
Inquiries continue, but police say they are not looking for anyone else in connection with the incident.
:: Anyone with information concerning this incident is asked to call police on 101 quoting CAD 3183/15Aug or Crimestoppers anonymously on 0800 555 111.