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 has declared 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.