The woman, identified in the media as Lisa Griffin, did not have her contract renewed in 2009 after she refused to undergo the test, which is not required of ethnic Korean teachers.
She maintained the mandatory test was "discriminatory and an affront to her dignity".
Griffin's employers, the Uslan Metropolitan Office of Education, had said that HIV/AIDS tests were "viewed as a means to check the values and morality of foreign English teachers," a UN statement said.
South Korean nationals in equivalent jobs were not required to do so.
South Korea has said it scrapped the HIV/AIDS tests for expatriate teachers in 2010.
The Geneva-based Committee on the Elimination of Racial Discrimination said the HIV/AIDS test "does not appear to be justified on public health grounds or any other ground and is a breach of the right to work without distinction to race, colour, national or ethnic origin."
It also said South Korea should "counter any manifestations of xenophobia, through stereotyping or stigmatising, of foreigners by public officials, the media and the public at large", and gave the country 90 days to inform the committee of the steps it has taken.
