The woman, 19, was found at City Warehouse ApartHotel on Great Ancoats Street, Manchester, at about 06:30 GMT.
Greater Manchester Police said they are concerned about any others who may have taken the pills and have advised them to seek medical attention.
Supt Stephen Howard said: "Even if you took it some hours ago this pill could be seriously harming your health."
A teenager has been found dead in a city centre hotel after apparently taking ecstasy, police have said.