Thomas D. Mangelsen (born January 6, 1946) is an American nature and wildlife photographer and conservationist. He is most famous for his photography of wildlife in the Greater Yellowstone Ecosystem, as he has lived inside the zone in Jackson, Wyoming, for over 40 years. In 2015, he and nature author Todd Wilkinson created a book, The Grizzlies of Pilgrim Creek, featuring a grizzly bear known as Grizzly 399, named so due to her research number. He has been active in the movement to keep the Yellowstone area grizzly bears on the Endangered Species List. Mangelsen is also known for trekking to all seven continents to photograph a diverse assortment of nature and wildlife. A photograph he took in 1988, Catch of the Day, has been labeled "the most famous wildlife photograph in the world". In May 2018, he was profiled on CBS's 60 Minutes. He has received dozens of accolades throughout the decades.