Christie, 58, called it an "important precautionary measure" due to his history of asthma and said he is "feeling good" but has mild symptoms. He said he is checked in at Morristown Medical Center in New Jersey.
The former governor announced he had tested positive for COVID-19 just hours before saying he moved to a hospital.
Christie had been with President Donald Trump for several days last week helping him prepare for the first presidential debate against Democratic challenger Joe Biden, who has tested negative for the coronavirus. He said "no one was wearing masks" during the sessions.
"I just received word that I am positive for COVID-19," Christie wrote. "I want to thank all of my friends and colleagues who have reached out to ask how I was feeling in the last day or two. I will be receiving medical attention today and will keep the necessary folks apprised of my condition."
Christie did not immediately respond to a message seeking comment Saturday. Hesaid Friday as he awaited results that he planned to quarantine at home for two weeks.
Christie was also one of the roughly 200 participants at an outdoor gathering at the White House to announce the nomination of Amy Coney Barrett to the U.S. Supreme Court that is considered a possible "super spreader event." Like most other attendees, Christie was not wearing a mask and was seen on video shaking hands and hugging other people.
Several other people who were at the ceremony have tested positive: Kellyanne Conway, the New Jersey native and former White House counselor; Republican Sens. Mike Lee of Utah; Thom Tillis of North Carolina; and Rev. John Jenkins, president of the University of Notre Dame.
Bill Stepien, Christie's former gubernatorial campaign manager now running Trump's re-election bid, also tested positive for the virus.