Gustav Heinrich Theodor Eimer (1843-1898) was a German zoologist.

In 1875, he became a professor of zoology and comparative anatomy at the University of Tübingen.

He is credited with popularizing the term "Orthogenesis" to describe an intrinsic drive in life towards perfection, a form of directed evolution. The "Eimer's organs" found in members of the mole family, especially in the Star-nosed Mole, are named after him. He described these organs in the European mole in 1871. Eimeria , a genus of parasitic protozoa, was also named after him.