Lloyd Russell-Moyle, a member of Parliament for the opposition Labour Party, is believed to be the only sitting lawmaker in the House of Commons to have disclosed that he has the virus.
Lloyd Russell-Moyle said he was diagnosed 10 years ago.
"Next year, I'll be marking an anniversary of my own: 10 years since I became HIV-positive," the 32-year-old told the lower house. "It's been a long journey -- from the fear to acceptance and, from today, advocacy, knowing my treatment keeps me healthy and that it protects any partner I have."
Taking medication for a number of years has made him "what the [National Health Service] calls HIV-positive 'undetectable,' " he said.
Russell-Moyle, who was elected in 2017, explained that means he does not get sick and "can't transmit HIV to someone else."
"So as the virus lays undetectable and dormant in my body; my medication ensures that the virus doesn't reactivate, doesn't progress and can't be passed on. That's why the NHS says: Undetectable equals un-transmittable."
The MP told the Press Association news agency that he "had a duty as a Member of Parliament" to make the disclosure.
He said government cuts to s*xual health budgets meant he could not "keep quiet" any more about an issue that "not only affects people I know, but it affects me so personally." The Department of Health and Social Care declined to comment.
Russell-Moyle added that he chose the timing of his announcement to mark the 30th World AIDS Day on December 1.
He is the second lawmaker to disclose that he is living with HIV. Former Labour Cabinet minister Chris Smith made the first revelation in 2005.
When someone is HIV-positive, regular treatment can nearly eliminate their chance of spreading the infection to others during s*x.
Also Thursday, Public Health England reported that the number of people newly diagnosed with HIV in the UK fell 17% between 2016 and 2017.
An estimated 92% of the 102,000 people living with HIV in Britain have been diagnosed, the agency said. Of those, 98% are on treatment, and 97% of those on treatment are virally suppressed, meaning HIV is undetectable in their blood.
The figures make the UK one of the first countries to reach the UN's 90-90-90 targets: 90% of people with HIV will have a diagnosis by 2020, 90% of those will receive antiretrovirals, and 90% of those will have viral suppression.
About 39 million people were living with HIV/AIDS worldwide in 2017, according to UNAIDS, the United Nations program that sets international targets.