Johannes Gessner (18 March 1709 – 6 May 1790) was a Swiss mathematician, physicist, botanicus, mineralogist and physician. He is seen as the founder of the "Naturforschende Gesellschaft in Zürich".
Gessner was born and died in Zürich, where he trained in Zürich with Johannes von Muralt. He moved to Basel to study medicine, continuing his studies in 1726 and 1727 at the University of Leiden. There he became a friend of Albrecht von Haller. With Haller he made a grand tour to Paris to finish their medicine studies. That is where he wrote his diary that was later published as Pariser Tagebuch. The two friends in 1728 studied mathematics under Johann Bernoulli and travelled through Switzerland.
Gessner became a doctor in Bazel in 1730, but he quickly chose a scientific career. In 1733 he became math professor and in 1738 he began to teach physics in Zürich. Gessner influenced many Swiss students, such as Johann Heinrich Rahn and Johann Georg Sulzer.
Gessner was a follower of Carolus Linnaeus and published over Swiss flora.
- Phytographia sacra, 1759–69
- Tabulae phytographicae, 1795–1804
- Pariser Tagebuch, 1727