Theodor Hahn (19 May 1824 – 3 March 1883) was a German hydrotherapist, naturopath, writer, and vegetarianism activist.

[3] Hahn operated a naturopathic sanatorium Auf der Waid in Oberwaid near St. Gallen from 1862.

[3] Medical Historian Karl Eduard Rothschuh commented that Hahn "started out exclusively with the water cure but with his addition of dietetics and vegetarianism to nature cure, he pushed its influence into the beginning of the health and life reform movement.

"[4] Hahn was influenced by Christoph Wilhelm Hufeland's book Makrobiotik and began to prescribe to his patients a lacto-vegetarian diet from 1852 that consisted of whole wheat bread, milk and uncooked vegetables.

[3] He argued for a meatless diet and provided evidence of its physiological benefits, including longevity and physical strength.