A patient infected in Dallas, Texas, is likely to have been infected by sexual contact, the Centers for Disease Control (CDC) told the BBC.
The person had not travelled to infected areas but their partner had returned from Venezuela.
Zika is carried by mosquitoes and has been linked to thousands of babies being born with underdeveloped brains.
It is spreading through the Americas and the World Health Organization (WHO) has declared the disease linked to the virus a global public health emergency.
The American Red Cross has meanwhile urged prospective blood donors returning from Zika-hit countries to wait at least 28 days before donating their blood.
The “self-deferral” should apply to people returning from Mexico, the Caribbean or Central or South America during the past four weeks, the Red Cross said in a statement.
- Two cases of the Zika virus have been confirmed in Australia. Officials said the two Sydney residents had recently returned from the Caribbean.
- Zika has also been found in two unrelated cases in the Republic of Ireland, officials there said. A man and an older woman, who have both recovered, had a history of travelling to a Zika-affected country.
- The UK has announced people will not be able to donate blood for 28 days after returning from a Zika-affected country.
Meanwhile, Brazil – the country worst hit by the outbreak – has revealed it is investigating 3,670 suspected cases of microcephaly in babies linked to the Zika virus.
A total of 404 cases have so far been confirmed – up from 270 last week – while 709 cases have been discarded, the country’s health ministry said.
The ministry also said 76 infant deaths from microcephaly, either during pregnancy or just after birth, were suspected.