Tom Frieden, director of the US Centres for Disease Control and Prevention (CDC), told a press conference that the patient is an adult who flew from Liberia to Texas to visit family earlier this month, Xinhua reported.
The patient had no symptoms when leaving West Africa but began to develop symptoms about four days after arriving in the US Sep 20, Frieden said.
He said the patient was admitted into a hospital in Texas and placed on isolation Sunday and that a CDC laboratory confirmed the positive result Tuesday.
"I have no doubt that we will control... this case of Ebola so it does not spread widely in this country," Frieden said.
Frieden noted that it's possible that those who had contact with this patient, including family members, may develop Ebola in the coming weeks but "there is no doubt, in my mind, that we will stop it here".
He said the US will identify all people who might have had contact with the patient.
The White House said in a statement that President Barack Obama and Frieden had discussed "stringent isolation protocols" to mitigate the risk of additional cases.
"Frieden noted that the CDC had been prepared for an Ebola case in the United States, and that we have the infrastructure in place to respond safely and effectively," the statement said.
US hospitals have treated five Ebola patients but they were all infected with the deadly virus in West Africa and then brought home for treatment.