The variant was found in a man in his 20s near Denver, Colorado.
State Governor Jared Polis announced the finding on Tuesday.
The patient is currently in isolation and has no travel history, state health officials said.
Scientists believe the new virus variant is more contagious than previously identified strains, but approved vaccines currently being rolled out are thought to be effective.
US public health officials are investigating other potential cases and performing contract tracing to determine the spread of the variant throughout the state.
"The health and safety of Coloradans is our top priority, and we will closely monitor this case, as well as all Covid-19 indicators, very closely," Gov. Polis said.
Gov. Polis and state health officials are expected to address the public Wednesday.
The discovery of the new variant lead America's Center for Disease Control to issue new rules on Christmas Day for travellers arriving to the US from the UK.
Arrivals in the States must now show proof of a negative Cove test.
Countries around the world have taken similar measures to try and prevent the spread of the strain.