Thomas Eugene Lovejoy III (August 22, 1941 – December 25, 2021) was an American ecologist who was President of the Amazon Biodiversity Center, a Senior Fellow at the United Nations Foundation and a university professor in the Environmental Science and Policy department at George Mason University. Lovejoy was the World Bank's chief biodiversity advisor and the lead specialist for environment for Latin America and the Caribbean as well as senior advisor to the president of the United Nations Foundation. In 2008, he also was the first Biodiversity Chair of the H. John Heinz III Center for Science, Economics and the Environment to 2013. Previously he served as president of the Heinz Center since May 2002. Lovejoy introduced the term biological diversity to the scientific community in 1980. He was a past chair of the Scientific Technical Advisory Panel (STAP) for the Global Environment Facility (GEF), the multibillion-dollar funding mechanism for developing countries in support of their obligations under international environmental conventions.

Lovejoy enrolled at Yale University, earning his bachelor’s degree in biology in 1964 while working as a zoological assistant at the Yale Peabody Museum of Natural History. He also received his Ph.D. in biology from Yale University.

As a tropical biologist and conservation biologist, Lovejoy has worked in the Amazon of Brazil since 1965.

From 1973 to 1987 he directed the conservation program at World Wildlife Fund-U.S., and from 1987 to 1998 he served as assistant secretary for environmental and external affairs for the Smithsonian Institution in Washington, D.C., and in 1994 became counselor to the secretary for biodiversity and environmental affairs. From 1999 to 2002, he served as chief biodiversity adviser to the president of the World Bank. In 2010 and 2011, he served as chair of the Independent Advisory Group on Sustainability for the Inter-American Development Bank. He was senior adviser to the president of the United Nations Foundation, chair of the Yale Institute for Biospheric Studies, and was past president of the American Institute of Biological Sciences, past chairman of the United States Man and Biosphere Program, and past president of the Society for Conservation Biology.

Lovejoy developed the debt-for-nature swaps, in which environmental groups purchase shaky foreign debt on the secondary market at the market rate, which is considerably discounted, and then convert this debt at its face value into the local currency to purchase biologically sensitive tracts of land in the debtor nation for purposes of environmental protection.

Critics of the 'debt-for-nature' schemes, such as National Center for Public Policy Research, which distributes a wide variety of materials consistently justifying corporate freedom and environmental deregulation, aver that plans deprive developing nations of the extractable raw resources that are currently essential to further economic development. Economic stagnation and local resentment of "Yankee imperialism" can result, they warn. In reality, no debt-for-nature swap occurs without the approval of the country in question.

Lovejoy has also supported the Forests Now Declaration, which calls for new market-based mechanisms to protect tropical forests.

Lovejoy played a central role in the establishment of conservation biology, by initiating the idea and planning with B. A. Wilcox in June 1978 for The First International Conference on Research in Conservation Biology, that was held in La Jolla, in September 1978. The proceedings, introduced conservation biology to the scientific community.

Lovejoy founded the Biological Dynamics of Forest Fragments Project (BDFFP) near Manaus, Brazil, in 1979 to understand the effects of the fragmentation on tropical rainforests on ecosystems and wildlife.

Lovejoy served on many scientific and conservation boards and advisory groups, is the author of numerous articles and books. As often misassociated, he is not the founder but served as an advisor in the early days of the public television series NATURE. He is no longer part of the creative team. He has served in an official capacity in the Reagan, George H. W. Bush, and Clinton administrations.

Lovejoy predicted in 1980 (see quote below), that 10–20 percent of all species on earth would have gone extinct by the year 2020.