Boulderthon: Boulder Marathon & Half Marathon

Boulderthon is one of the top-rated marathons in the United States Tucked into a picturesque valley below the iconic Flatirons, Boulder, Colorado sets the backdrop for the top-ranked, signature Boulder Marathon, Boulderthon. Boulder is a world-class destination offering 300 days of sunshine and endless recreation, shopping, dining, cultural and entertainment choices. It's no wonder National Geographic named Boulder the Happiest City in America. Join us on for the largest September running weekend in the U.S., on Sunday, September 28th, 2025 for the fifth annual Boulderthon Marathon in stunning Boulder, Colorado. The course features an exciting, all-new Start downtown, then leads you through the beautiful back roads of Boulder, Colorado at the foothills of the Flatirons, before winding you to Downtown Boulder for an epic, one-of-a-kind finish on the historic Pearl Street Mall! Boulderthon is the perfect fall race to enjoy Boulder, Colorado, named the happiest & fittest city in America. Boulderthon has doubled in size every year since its inception, selling out each year, and is expected to sell out again. Boulderthon is a Boston Marathon Qualifier.