State Department senior adviser Philippe Reines said Clinton was being treated with anti-coagulants at New York-Presbyterian Hospital, NBC News reported. She is to be monitored at the hospital for 48 hours, he said.
"Her doctors will continue to assess her condition, including other issues associated with her concussion," he said. "They will determine if any further action is required."
CNN reported Reines said the clot was found by doctors during a follow-up exam related to the concussion she suffered early this month, her spokesman said.
Clinton, 65, who is leaving the Obama administration this year, was injured when she fainted due to dehydration related to a stomach virus.