Nantucket Cottage Hospital spokesman Noah Brown says she was admitted Sunday, but would not say why she was admitted.
Heinz Kerry, 74, was later transferred to Massachusetts General Hospital in Boston, spokesman Glen Johnson said in a statement.
Sources close to the family told ABC News that Heinz Kerry was taken from her home to the hospital by ambulance after falling ill in the afternoon. They did not elaborate on her illness. Johnson said John Kerry accompanied his wife during both transports to Nantucket and Massachusetts General Hospital.
"The family is grateful for the outpouring of support it has received and aware of the interest in her condition, but they ask for privacy at this time," Johnson's statement read.
Heinz Kerry is a breast cancer survivor. She was diagnosed in 2009.
Heinz Kerry is also the widow of former Sen. John Heinz, heir to the Heinz ketchup fortune. She married John Kerry in 1995.
ABC News and the Associated Press contributed to this report.