In the post, the couple detailed how Harry hoped to help eradicate the stigma of the illness.
"This morning, The Duke of Sussex joined forces with @gareththomasofficial and The Terrence Higgins Trust to raise awareness for National HIV Testing Week, which takes place from November 16th-22nd," they wrote.
"The Duke has been a longtime advocate of HIV awareness, and of eradicating the stigma surrounding the virus - an extension of his mother’s legacy.
"Not enough people know that thanks to the work of so many over the years, HIV is no longer a death sentence and testing for it is quick and easy.
"Today the Duke shared the magnitude of having someone like Gareth Thomas, one of our most legendary Welsh rugby players, demonstrate that you can be HIV positive and live a healthy, strong, fulfilling life.
"Gareth is an excellent role model for anyone living in fear, and an amazing example of both physical and mental strength.
"An estimated 1 in 14 people in the UK live with HIV and are still undiagnosed. Please get tested and take control of your life and your health - even if you don’t feel you’re at risk, getting tested will help smash the stigma and normalise it for others," they continued in their post.
Before signing off with details of how to find out more, or get tested for the virus: "To find out more about how to get tested or National HIV testing week, please visit @ThtOrgUK."
Prince Harry has taken two public HIV tests in the past to raise awareness of the condition.
Gareth, who had been greeted by Harry giving him a hug, said at the event: “Stigma is this real thing that affects people in very negative ways.
“I think someone like Harry has a way bigger platform than any of us and he can reach parts of the world where people are still very much living in shame.”
Gareth also revealed that the pair had spoken after the Duke of Sussex sent a message of support to the rugby legend.
Prince Harry’s son Archie also received an adorable gift from the meeting in the shape of a Harlequins top with the six-month-old’s name on the back.