Sinema, who trained for a year, finished Ironman Arizona's 2.4-mile swim, 112-mile bike ride and 26.2-mile run Sunday in 15 hours and 12 minutes.
She told Politico last winter she was competing in the triathlon with friends with whom she has competed in marathons.
Sinema, 37, is the first active member of Congress to compete in an Ironman, several media outlets, including Triathlete Magazine, reported.
Sinema did the race to support Desert States Charitable Foundation, a charity that provides scholarships and support to area children, and the Leukemia and Lymphoma Society.
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend
Pregnant Mila Kunis wins 'Best Villain' at MTV Movie Awards