Ernst Johann Otto Hartert (October 29, 1859 - November 11, 1933) was an German ornithologist and oologist. Hartert was born in Hamburg. He was employed by Lionel Walter Rothschild as ornithological curator of his private museum at Tring from 1892 to 1929.

Hartert published the quarterly museum journal Novitates Zoologicae (1894-1939) with Rothschild, and the Hand List of British Birds (1912) with Jourdain, Ticehurst and Witherby. He wrote Die Vögel der paläarktishen Fauna (1903-22) and travelled in India, Africa and South America on behalf of his employer.

In 1930, Hartert retired to Berlin, where he died.