Great Ormond Street Hospital said Wednesday that Hope Williams died Tuesday night after the surgery. The hospital says her sister, Faith, is in stable condition.
The twins were born last week, joined at the abdomen, to 18-year-old Laura Williams.
Twins who share an abdomen usually share the liver, bile ducts from the liver, and parts of the intestines.
At least 40 doctors, nurses and other health professionals were involved in the separation of the Williams twins in London.
Stephen Cox, a spokesman for the Great Ormond Street Hospital for Children in London, said the decision to separate Faith and Hope Williams was brought forward after concerns developed Monday night. Cox would not specify what the concerns were.
The twins were born last week to Laura and Aled Williams. Doctors originally said they would have preferred to wait to separate the twins until they were older. The final decision to operate was made by the family.
The baby girls were joined at the abdomen. They were said to share a liver and intestines.
Conjoined twins occur about once in every 50,000 live births in the United States.
Twins joined at the chest are easier to separate than those joined elsewhere, like at the head. Last month, doctors in Cleveland decided it would have been too risky to separate 4-year-old twins born in Italy, connected at the head.
There were at least 40 doctors, nurses and other health professionals involved in the separation of the Williams twins in London.