Leonhard Hess Stejneger (October 30, 1851 - February 28, 1943) was a zoologist.

Stejneger was born in Bergen, Norway, and studied law and philosophy at the University of Christiania. He got a Ph.D. and started a brief career as a lawyer. However, his naturalist interests proved stronger and in 1881 he moved to the United States and started working at the Smithsonian under Spencer Fullerton Baird.

He participated in numerous expeditions to the north of the North American continent. From 1882 to 1883 he was sent on an exploration mission to Bering Island and Kamchatka. In 1895 he went to the Commander Islands, studying fur seals for the U.S. Fish Commission. He returned there a second time in 1922.

Within the Smithsonian, he moved up the career ladder. In 1884 he was Assistant Curator for birds, in 1889 he became Curator for reptiles, in 1899 Curator for reptiles and amphibians, and from 1911 on he was the Head Curator for biology, a post he held until his death, having been exempted from retirement by a presidential decree.

He published more than 400 scientific works on birds, reptiles, seals, the herpetology of Puerto Rico, and other topics.

During his trip to Bering Island, he became fascinated by the life of Georg Wilhelm Steller, an 18th century naturalist who had visited the island before. He researched the life of Steller very thoroughly over the next few decades, a hobby which culminated in his only non-scientific publication, an authoritative biography of Steller.