Officials at Texas Health Presbyterian Hospital say the unidentified patient is being kept in isolation.
The man is thought to have contracted the virus in Liberia before travelling to the US nearly two weeks ago.
More than 3,000 people have already died of Ebola in West Africa and a small number of US aid workers have recovered after being flown to the US.
“An individual travelling from Liberia has been diagnosed with Ebola in the United States,” Centers for Disease Control and Prevention Director Thomas Frieden told reporters on Tuesday.
According to Mr Frieden, the unnamed patient left Liberia on 19 September and arrived in the US the next day to visit relatives, without displaying any symptoms of the virus.
Symptoms of the virus became apparent on 24 September, and on 28 September he was admitted to a Texas hospital and put in isolation.