Emergency services were called to reports of a woman with stab injuries at an address in Landseer Avenue in Manor Park, Newham, just before 10am on Saturday.
Despite the best efforts of officers and paramedics, she was pronounced dead at the scene.
Police have launched a murder investigation and a man in his 30s, thought to be known to the woman, has been arrested.
The woman’s family have been informed.
A Met Police statement said: “A murder investigation has been launched and a man arrested after a woman was fatally stabbed in Manor Park.
“Police were called at 9.56am on Saturday, April 2, to a report of a woman with stab injuries at an address in Landseer Avenue, E12.
“Officers attended along with London Ambulance Service and London’s Air Ambulance and found a critically injured woman aged in her 80s.
“She was pronounced dead at the scene.
“The woman’s family have been informed.
“A post-mortem examination will be arranged in due course.
An urgent investigation was launched and officers searched the surrounding area.
“A man in his 30s was detained nearby a short time later.
“He has been taken into police custody.
“At this early stage, it is thought that the woman and the arrested man were known to each other.
“Inquiries are ongoing to establish the circumstances.
“The Met’s investigation is led by detectives from Specialist Crime (Homicide).”
Anyone with information is asked to call 101 quoting ref 2126 / 02apr.