Marcus Elieser Bloch (1723–1799) was a German physician and naturalist who is best known for his contribution to ichthyology through his multi-volume catalog of plates illustrating the fishes of the world.

Brought up in a Hebrew-speaking Jewish family, he learned German and Latin and studied anatomy before settling in Berlin as a physician.

He is generally considered one of the most important ichthyologists of the 18th century, and wrote many papers on natural history, comparative anatomy, and physiology.

He then became a general practitioner in Berlin and married Breinche, daughter of "protected Jew" Ruben Joseph Rintel (1699–1765?)

The first three volumes describe fishes in Germany and were entitled Oeconomische Naturgeschichte der Fische Deutschlands, the remaining volumes (in 9 parts) dealt with fishes from other parts of the world and were entitled Naturgeschichte der ausländischen Fische.

A French translation "Ichthyologie, ou histoire naturelle, générale et particulière des poissons" was published in 12 volumes between 1785 and 1797.

Bloch's collection of about 800 surviving specimens is preserved at the Museum für Naturkunde of the Humboldt University of Berlin.