Thomas, who took over from Brian O'Driscoll as captain of the 2005 Lions, became the first high-profile rugby player to come out as a gay man last month.
The Cardiff Blues wing cum centre will be a figurehead for LGBT History Month, which is celebrated every February and will this year focus on making LGBT people more visible in sport in 2011 and 2012.
"I feel honoured to have been asked to be the patron of LGBT History Month," said Thomas, who started four games for the 2005 Lions, including all three Tests against the All Blacks.
"I don't want to just make this a token role however; I intend to be hands on within my role, not only to make a difference to others, but to also grow myself."
As the first Welshman to reach 100 international caps and as a former Wales captain during the 2005 Grand Slam winning season, Thomas is one of the most well respected figures in the national game.
Unsurprisingly, the organizers of February's LGBT event are hopeful that he can play an important role in riding sport of homophobia.
"Gareth Thomas is a legend in the world of rugby. For someone of his stature to come out will be of enormous benefit to young players," said LGBT History Month co-chair Tony Fenwick.
"I deeply admire his honesty and his genuine commitment to kick homophobia into touch. We're already talking about an exciting project.
"We're honoured to have him on board as a patron. He's already showing himself to be a respected role model and we look forward to working with such a wonderful ambassador."