Clinton (65) was admitted to New York Presbyterian’s Washington Heights complex, where she will stay for at least 48 hours to be closely monitored.
Doctors will treat the former first lady with anticoagulants, said her spokesman, Philippe Reines.
“Her doctors will continue to assess her condition, including other issues associated with her concussion,” Reines said. “They will determine if any further action is required.”
He would not elaborate on the location of the clot.
Clinton was suffering from a stomach virus earlier this month when she fainted and hit her head, causing the concussion, while alone at her home. Doctors said she was dehydrated at the time.
Clinton is known for a gruelling travel schedule that has already made her the most-travelled secretary of state in history, with visits to 112 countries while in the job.
Clinton plans to step down as America’s top diplomat next month. US President Obama has nominated Senator John Kerry to replace her.