The 23-year-old's publicist, Claire Mercuri, said Saturday that Joel is in stable condition and receiving treatment.
She declined to say why she had been hospitalized.
"We are currently assessing her needs," she said.
Joel lives in Manhattan and had performances scheduled in the city next week.
Fire Department records indicate that paramedics responded to the West Village building where Joel lives at 12:21 p.m. Saturday and transported one woman to St. Vincent's Hospital. The fire department declined to identify the patient.