Dean Karnazes (English: /ˌkɑːrnɛˈzɪs/ car-NEH-zis; born Constantinos Karnazes; August 23, 1962), is an American ultramarathon runner, and author of Ultramarathon Man: Confessions of an All-Night Runner, which details ultra endurance running for the general public.

[6] In 2006 he said that he remained close to the friends he made at San Clemente High School, which was also attended by both his siblings.

[4] Growing up in the city of San Clemente gave him a love of the outdoors, and an appreciation of its small-town feel.

As Karnazes grew older, he began testing his limits: by age eleven he had hiked rim-to-rim across the Grand Canyon and had climbed Mount Whitney, the highest mountain in the contiguous United States; for his 12th birthday, he cycled 40 miles (64 km) to his grandparents' home without telling his parents.

Using this motto as a basis, that season Karnazes won the 1-mile (1.6 km) California State Long-Distance Championship held on the Mt SAC track.

Karnazes also ran his first endurance event that year, a fundraising run on a track for underprivileged children, finishing in just under six hours and raising a dollar a lap from his sponsors.

Since marathon races are typically held only on weekends, on the other days Karnazes (accompanied by between one and 50 runners) ran the course of a marathon in each state using the help of the race director and staff of each event to officially run the certified course, but on a different day than the "live" event.

[24] Karnazes' achievements have been derided among some runners as tainted with hyperbole and that his fame is primarily due to self-promotion in the media, rather than top athletic performances.

[29] The "Ultra Marathon Man" episode of Stan Lee's Superhumans documentary television series maintained that Karnazes is able to reduce the build-up of lactic acid over long periods of time.

[3] Karnazes attended California Polytechnic State University at San Luis Obispo, where his major subject was food science technology.

[4] Karnazes' children accompanied their father for much of his run of 50 marathons in 50 states, as they were home-schooled at the time.

[34] On August 12, 2022, Karnazes claimed in an Instagram video to have been attacked by a coyote during a race in the Marin Headlands.

[35] National Park Service rangers noted that his story did not match biologist's understanding of coyote behavior, even among those habituated to humans, and that his injuries appeared to be sustained from a fall, rather than an animal attack.