Essex Police said officers were called to an address in Retort Close, Southend-on-Sea by the ambulance service at 23:30 BST on Monday.
A woman suffered "serious injuries" and died at the Royal London Hospital on Wednesday, the force said.
It added the dog, which was not a banned breed, had been seized.
Police said officers were not looking for anyone else in connection with the incident and a file would be prepared for the coroner in due course.
"Our deepest sympathies are with the family of the woman involved at this time," Det Supt Gary Biddle said.
"While our investigation into the circumstances of this tragic incident continues, we would ask that the privacy of the woman's family is respected."
the bible predicted there would be a time when creatures would attack humans and it seems to be happening not only with dogs but otters off the coast of Santa Cruz, CA, orcas at the Strait of Gibralter, bears in Japan... yet dogs are killing more people across the board.