Alexander von Frantzius (10 June 1821 – 18 July 1877) was a German physician and naturalist.
Frantzius collected natural history specimens in Brazil from 1849–1853 and Costa Rica in 1853, many of which he sent to Jean Cabanis at the Berlin Museum. He set up a pharmacy in San José in Costa Rica, employing José Castulo Zeledón as his assistant. He returned to Germany in 1868.
He is commemorated in the names of a number of animals, including the Prong-billed Barbet Semnornis frantzii, the Fiery-billed Aracari Pteroglossus frantzii, the Mountain Elaenia Elaenia frantzii and the Ruddy-capped Nightingale-thrush Catharus frantzii.