Ladan Bijani died on Tuesday shortly after a historic operation to separate her from her twin sister Laleh, hospital officials announced in Singapore.
Laleh is in critical condition in Raffles Hospital, where the surgery started on Sunday morning.
Ladan died from severe loss of blood after the separation was completed. Laleh had also lost a lot of blood, and doctors are concentrating their efforts on stabilising her.
Well-wishers keeping vigil at the hospital burst into tears when the news was announced.
Raffles Hospital spokesman Prem Kumar Nair had, just moments before, announced that the women were in a critical condition.