The reality TV star was taken to the Cedars-Sinai Medical Center in
Los Angeles after complaining of feeling unwell and doctors quickly
diagnosed her as suffering from the illness.
Her representative said: "Nicole has checked into Cedars-Sinai where she is being treated for pneumonia. She is doing well."
Over the weekend, Nicole - who has two children, 22-month-old Harlow and two-month-old Sparrow with partner Joel Madden - had spoken about not feeling well.
She wrote on her Twitter page: "OK cold, it`s been six days. Let`s part ways graciously and keep it moving."
Earlier this week, Nicole was granted a restraining order against two paparazzi who she claims caused her recent car accident.
The 28-year-old socialite appeared in court to testify how the two male photographers, Eduardo Arrivabene and Ivon Miguel, had been hounding her constantly.
The judge ruled the two men are to stay 50 yards away from Nicole and her two children for the next three years.