Experts say it frees women from relying on men to wear condoms and allows them to protect themselves confidentially.
Dr Anthony Fauci, the director of the US National Institute of Allergy and Infectious Diseases, told the BBC: "If you can give women the opportunity to protect themselves in a way that is completely confidential - that's a long and big step to helping them.
"In societies where women are, unfortunately but true, somewhat second-class citizens, that makes women extremely vulnerable to getting infected with HIV."
The flexible ring, similar in size to the contraceptive diaphragm, releases an antiretroviral drug called dapivirine for a month.
But scientists were unsure it would work in teenagers, who can be notoriously difficult when it comes to health advice.
The six-month US trial gave the ring to 96 sexually active girls aged 15 to 17, who had not used it before.
Data presented at the IAS Conference on HIV Science, showed:
87% of the girls had detectable levels of the drug in their vagina
95% said the ring was easy to use
74% said they did not notice the ring in day-to-day life
There were some concerns before the trial that the girls' partners would not like the feel of the ring, but it reportedly enhanced pleasure.