Theodor Hartig (21 February 1805 – 26 March 1880) was a German forestry biologist and botanist.

[2][3] The Hartig net is part of the structure of ectomycorrizae, mutualistic symbioses between fungi and plant roots.

In collaboration with his father, Georg Ludwig Hartig, he published the work entitled, Forstliches und naturwissenschaftliches Konversationslexikon.

The eleventh edition of his father's Lehrbuch für Förster, the later reprints of which he had revised, was published in 1877.

He was the son of Georg Ludwig Hartig (1764–1837), a German forester.