Theodosius Grigorievich Dobzhansky (Russian: Феодосий Григорьевич Добржанский; Ukrainian: Теодосій Григорович Добржанський; January 25, 1900 – December 18, 1975) was a Russian-born American geneticist and evolutionary biologist. He was a central figure in the field of evolutionary biology for his work in shaping the modern synthesis and also popular for his support and promotion of theistic evolution as a practicing Christian. Born in the Russian Empire, Dobzhansky immigrated to the United States in 1927 at the age of 27.

His 1937 work Genetics and the Origin of Species became a major influence on the modern synthesis. He was awarded the U.S. National Medal of Science in 1964 and the Franklin Medal in 1973. He is famous for his essay "Nothing in Biology Makes Sense Except in the Light of Evolution."

Dobzhansky was born on January 25, 1900, in Nemirov, Russian Empire (now Nemyriv, Ukraine), the only child of Grigory Dobzhansky, a mathematics teacher, and Sophia Voinarsky. He was given an unusual name, Theodosius, because he was born after his middle-aged parents prayed for a child to St. Theodosius of Chernigov. In 1910 the family moved to Kiev.

At high school, Dobzhansky collected butterflies and decided to become a biologist. In 1915, he met Victor Luchnik who convinced him to specialize in beetles instead. Dobzhansky attended the University of Kiev, where he then studied until 1924 specializing in entomology. He then moved to Leningrad (today St. Petersburg) to study under Yuri Filipchenko, where a Drosophila melanogaster laboratory had been established.

On August 8, 1924, Dobzhansky married geneticist Natalia "Natasha" Sivertzeva, who was working with Ivan Schmalhausen in Kiev. The Dobzhanskys had one daughter, known under her married name as Sophie Coe, an anthropologist, food historian, and author, primarily known for her work on the history of chocolate.

Before immigrating to the United States, Dobzhansky published 35 scientific works on entomology and genetics.

Dobzhansky immigrated to the United States in 1927 on a work–study scholarship from the International Education Board of the Rockefeller Foundation. Upon arriving in New York City on December 27, he joined the Drosophila Group at Columbia University working alongside Thomas Hunt Morgan and Alfred Sturtevant. Their work provided crucial information on Drosophila cytogenetics. Additionally, Dobzhansky and his team helped establish Drosophila pseudoobscura, within the genus Drosophila, as a favorable model organism in evolutionary-biological studies ever since they published their influential works. Dobzhansky's original mindset (after studying alongside Yuri Filipchenko), was that there were serious doubts on using data obtained from phenomena happening in local populations (microevolution) and phenomena happening on a global scale (macroevolution). Filipchenko also believed that there were only two types of inheritance: Mendelian inheritance of variation within species, and Non-Mendelian inheritance of variation in a macroevolutionary sense. Dobzhansky later stated that Filipchenko "bet on the wrong horse".

He followed Morgan to the California Institute of Technology from 1930 to 1940. On the basis of his experiments, he articulated the idea that reproductive isolation can be caused by differences in presence of microbial symbionts between populations. In 1937, he published one of the major works of the modern evolutionary synthesis, the synthesis of evolutionary biology with genetics, titled Genetics and the Origin of Species, which amongst other things, defined evolution as "a change in the frequency of an allele within a gene pool". Dobzhansky's work was instrumental in spreading the idea that it is through mutations in genes that natural selection takes place. Also in 1937, he became a naturalized citizen of the United States. During this time, he had a very public falling out with one of his Drosophila collaborators, Alfred Sturtevant, based primarily in professional competition.