In a statement, the comedian's daughter Melissa Rivers said her mother was "resting comfortably" with family. She did not elaborate on the 81-year-old star's condition.
"We ask that you continue to keep her in your thoughts and prayers," she said and offered thanks for the "overwhelming love and support for my mother".
Sid Dinsay, spokesman for The Mount Sinai Hospital in New York, said Rivers was brought there yesterday morning.
New York City police sources said she was taken to the hospital just after 9.30am local time. It was unclear why she was visiting the doctor's office and Rivers' representatives did not immediately respond to requests for comment.
The entertainer has logged a half-century in show business and gave rise to red carpet commentary - and the sarcastic criticism that often accompanies it. Her signature red carpet query is: "Who are you wearing?"
She continues to maintain a busy schedule and was to perform at the Count Basie Theatre in Red Bank, New Jersey, today.
She is also due to play Glasgow Royal Concert Hall on Wednesday, October 22.
Rivers spoke at an employee event at Time Inc in New York on Wednesday night and appeared healthy, practically jogging when she walked in to take her seat, said Shira Blum, an online project manager.
"She seemed totally healthy," Ms Blum said. "She was very energetic, hilarious, funny. And it was such a shock, a surprise to hear the news this morning."
Rivers took questions and said she wakes up every morning and "is thankful that everything works," Ms Blum said.
An early and outspoken proponent of cosmetic surgery, Rivers' drastically altered her appearance over the years - and found plenty of material for jokes - "I've had so much plastic surgery, when I die they will donate my body to Tupperware."
Rivers, who hosts Fashion Police on E! network, also presides over an online talk show, In Bed With Joan, and co-stars with her daughter on the WEtv reality show Joan & Melissa: Joan Knows Best?
Her latest book, Diary Of A Mad Diva, was released this summer.
In 2009, Rivers emerged as the winner of NBC's The Celebrity Apprentice and a documentary, Joan Rivers: A Piece Of Work, premiered in cinemas in 2010.
New York native Rivers originally entered show business with the dream of a theatrical career, but comedy became a way to pay the bills while she auditioned for acting roles.
"Somebody said, 'You can make six dollars standing up in a club'," she said in 2013, "and I said, 'Here I go!' It was better than typing all day."
After proving herself in comedy clubs as a rarity - a woman comedian - Rivers was a smash on her first booking on The Tonight Show Starring Johnny Carson in 1965. "God, you're funny," Carson told her.