Carl Henry Eigenmann (March 9, 1863 – April 24, 1927) was a German-American ichthyologist of the late nineteenth and early twentieth centuries who, along with his wife Rosa Smith Eigenmann and his zoology students, is credited with identifying and describing for the first time 195 genera containing nearly 600 species of fishes of North America and South America. Especially notable among his published papers are his studies of the freshwater fishes of South America, the evolution and systematics of South American fishes, and for his analysis of degenerative evolution based on his studies of blind cave fishes found in parts of North America and in Cuba. His most notable works are The American Characidae (1917–1929) and A revision of the South American Nematognathi or cat-fishes (1890), in addition to numerous published papers such as "Cave Vertebrates of North America, a study of degenerative evolution" (1909) and "The fresh-water fishes of Patagonia and an examination of the Archiplata-Archelenis theory" (1909).

Eigenmann was an alumnus of Indiana University, an instructor/professor in IU's department of zoology, and an administrator at IU's Bloomington, Indiana, campus for more than forty years. He was also the first dean of the IU graduate school from 1908 to 1927. In addition to his duties at IU, Eigenman was honorary curator of fishes at the Carnegie Museum in Pittsburgh, Pennsylvania, from 1909 to 1918. Eigenmann was elected to the National Academy of Sciences in 1923. He was also a fellow of the American Association for the Advancement of Science, a member and past president of the Indiana Academy of Science, an honorary member of the California Academy of Sciences and of the Sociedad de Ciencias Naturales of Bogotá, Colombia, as well as a member of other scientific organizations, including Sigma XI, a science honorary, and Phi Beta Kappa.

Carl Henry Eigenmann was born on March 9, 1863, in Flehingen, Grand Duchy of Baden, to Philip and Margaretha (Lieb) Eigenmann. In 1877, at the age of fourteen, he emigrated to the United States with an uncle and settled in Rockport, Indiana.

In 1879, at the age of sixteen, Eigenmann enrolled at Indiana University in Bloomington, Indiana, intending to study law, Latin, and Greek. He became a member of Sigma Chi fraternity and then became interested in zoology after taking a biology course under David Starr Jordan. He then decided to pursue a career as an ichthyologist.

Eigenmann earned a bachelor's degree from IU in 1886, a master's degree in 1887, and a Ph.D. from IU in 1889. He also studied South American fish collections at Harvard University for a year in 1887–1888, before beginning his career as a researcher and educator in California.

While Eigenmann was still at student at IU, his first published paper, "Notes on skeletons of Etheostomatinae", co-authored with Jordan, was published in the Proceedings of the National Museum in 1885. "A review of the genera and species of Diodontidae found in American waters", the first of many papers that Eigenmann authored on his own, was published in the Annals of the New York Academy of Science in 1886, when he was twenty-three years old.

Through his IU professor, David Starr Jordan, Eigenmann met Rosa Smith, a former IU student from San Diego, California, who was already becoming known for her work on West Coast fishes. Eigenmann corresponded with Smith while she was living in San Diego, and traveled to California, where the couple married at Smith's home on August 20, 1887. Rosa Smith Eigenmann, also an ichthyologist, collaborated with her husband on several research projects after their marriage, but discontinued her own research pursuits in 1893, due to family responsibilities; however, she continued to edit her husband's research papers.

The Eigenmanns had five children. Lucretia Margaretha Eigenmann (1889–1965), the eldest, was mentally disabled; their son, Theodore Smith Eigenmann (1893–1970), was eventually institutionalized after serving in the army in 1918. The three other Eigenmann children pursued professional careers. Charlotte Elizabeth Eigenmann (1891–1959) graduated from Stanford University and pursued an editorial career. Adele Rosa (Eigenmann) Eiler (1896–1978) accompanied her father on the Irwin Expedition to South America in 1918–19 and received a medical degree from Indiana University in 1921. Adele later married John Oliver Eiler, and they moved to San Diego. Their young child, Thora Marie Eigenmann (1901–1968), a graduate of the University of Missouri, became a writer.