Dean Karnazes (ডিন কার্নাজেস) 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.

Karnazes was born in Inglewood, California to parents of Greek ancestry; Nick and Fran Karnazes. He had two siblings; brother Kraig and a sister, Pary, who died in an automobile accident at the age of 18.

His father worked as a field naturalist for the Orange County Department of Education in 2006. The younger Karnazes brother grew up in Diamond Bar, California and San Clemente, California. 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. Growing up in the city of San Clemente gave him a love of the outdoors, and an appreciation of its small-town feel. At the time his parents still lived in the house where he had grown up.

While attending kindergarten, Karnazes began running home from school; he took up running for fun.

At first, Karnazes ran direct routes from school to his home. Later, he began to run diversionary routes that would extend his run and take him into uncharted territory. By third grade, he was participating in and organizing short running events with other children. 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 for fun without telling his parents.

In junior high school, Karnazes met Jack McTavish, a track coach who became Karnazes' mentor and introduced him to the appeal of long-distance running. McTavish's basic running instructions were simple: "Go out hard and finish harder." 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. At the end of the race, Coach McTavish commented: "Good work son, how'd it feel?" To this Karnazes replied: "Well, going out hard was the right thing to do. It felt pretty good." The coach replied: "If it felt good, you didn't push hard enough. It's supposed to hurt like hell." A week after the race, Karnazes' father's job was transferred to San Clemente. These were the last comments the coach ever said to Karnazes, who has stated that he lives by these words to this day.

In 1976, as a high school freshman at San Clemente High, Karnazes joined the cross country team under Benner Cummings. Cummings' running theory was that running is about finding your inner peace; his motto was "run with your heart". That season, Karnazes was awarded "Most Inspirational" team member. 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. While most students ran only 10–15 laps around the track, he ran 105, a full marathon. When the cross-country season ended he signed up for the track team but did not get along with the coach. After walking off the track team he then stopped running for fifteen years.