The 45-year-old was cheered and applauded as he completed the mammoth challenge in 12 hours 18 minutes and 29 seconds, placing 413th out of 2,039 participants.
The visibly emotional ex-British and Irish Lions captain was supported by parents Yvonne and Barry as he took on the Ironman Wales race in the seaside resort of Tenby in south Wales on Sunday.
Thomas embraced husband Stephen at the finish line after vowing to “break the stigma” around the illness after blackmailers put him “through hell” threatening to expose his secret.
The sportsman completed the 2.4 mile swim in Carmarthen Bay in just under an hour-and-a-half, then a six-and-a-half hour, 112-mile cycle race across south Pembrokeshire, before an exhausting finish with a 26.2 mile marathon in four hours and 17 minutes.
Thomas was hailed a “legend” by the Duke of Cambridge for his “courageous” admission and praised by politicians such as Labour leader Jeremy Corbyn and Health Secretary Matt Hancock.
The ex-fullback, who came out as gay in 2009, is thought to be the first UK sportsman to go public about living with HIV.
In a video posted on Twitter on Saturday evening, he said: “I am living with HIV.
“Now you have that information that makes me extremely vulnerable, but it does not make me weak.
“Even though I have been forced to tell you this, I choose to fight, to educate and break the stigma around this subject.
“And that begins today when I take on the toughest ironman in the world in Tenby, and I push myself physically to the limit.”
Thomas said he went public with his illness after being put “through hell” by blackmailers.
In an interview with the Sunday Mirror, Thomas said: “I’ve been living with this secret for years.
“I’ve felt shame and keeping such a big secret has taken its toll.
“I was in a dark place, feeling suicidal. I thought about driving off a cliff.
“To me, wanting to die was just a natural thought and felt like the easier way out, but you have to confront things.”
The former Cardiff Blues player won 103 caps and scored 41 tries for Wales between 1995 and 2007, and he is 13th on the all-time international test try-scoring list.