The 46-year-old woman "likely acquired" human immunodeficiency virus while in a monogamous relationship with an HIV-positive female partner in Texas, said the Centers for Disease Control and Prevention.
The woman, whose name was not released, had engaged in heterosexual relationships in the past, but not in the 10 years prior to her HIV infection.
Her HIV-positive partner, a 43-year-old woman who first tested positive in 2008, was her only sexual partner in the six months leading up to the test that came back positive for HIV.
She did not report any other risk factors for acquiring the virus that causes AIDS, such as injection drug use, organ transplant, tattoos, acupuncture or unprotected sex with multiple partners.
The strain of HIV with which she was infected was a 98 percent genetic match to her partner's, said the CDC in its weekly report.
Authorities first learned of the case in August 2012 from the Houston Department of Health.
The couple said they had not received any counseling about safe sex practices, and reported that they routinely had sex without barrier methods.
"They described their sexual contact as at times rough to the point of inducing bleeding in either woman," said the CDC report.
"They also reported having unprotected sexual contact during the menses of either partner."
The partner who was infected since 2008 had been prescribed antiretroviral drugs in 2009 but stopped taking them in November 2010, and was lost to follow up in January 2011.
The CDC warned that although such cases are rare, "female-to-female transmission is possible because HIV can be found in vaginal fluid and menstrual blood."
People with HIV should be under the care of a doctor and take their prescribed medicines to keep their viral load down and reduce the risk of infecting a partner, the CDC said.
Very few cases of this kind have been documented, and confirmation "has been difficult because other risk factors almost always are present or cannot be ruled out," said the report.
One survey of 960,000 female blood donors found 144 who tested positive for HIV and were therefore blocked from donating.
Of 106 of those women who agreed to interviews, none described female-to-female sexual contact as their only risk factor.
The CDC also described one case of a woman in the Philippines who tested positive for HIV and said she had sex only with women and did not inject drugs, though no source of infection could be confirmed.
One other case is known of a 20-year-old woman who was diagnosed with HIV after a two-year monogamous relationship with a female partner who was HIV positive. While both women had the same drug-resistant HIV mutations, no tests were done to identify if their HIV strains were a genetic match.