Johann Wilhelm Fortunat Coaz (31 May 1822 – 18 August 1918) was a Swiss forester, topographer and mountaineer from Graubünden. In 1850 he made the first ascent of Piz Bernina, the highest mountain in the Eastern Alps. He also gave Piz Bernina its name, after the eponymous pass.
Coaz was born in Antwerp in 1822, the son of Wilhelm Johann Coaz, a professional officer, and his wife Salomé née Koehl. He died in Chur in 1918.
Between 1841 and 1843 he trained at the Royal Saxon Academy of Forestry in Tharandt to become a forester, then assumed the role of mountain topographer in Graubünden in the service of the Federal Topographic Bureau. When he was 28 he became private secretary to the topographer Guillaume-Henri Dufour. From 1851 to 1873 he was chief forestry inspector of the cantons of Graubünden and St. Gallen (1873–75). In 1875 Coaz became the first Federal Chief Forestry Inspector, a position he held until 1914.
Coaz was scientifically active in the areas of forest botany, topography, meteorology, and glacier and avalanche research. He described Larix X marschlinsii Coaz, noted in Krüssmann as a hybrid between L. kaempferi and L. russica that came from the Tscharnerholz Forest Nursery, near Morat, Switzerland in 1901 (the species is now considered a hybrid between L. decidua and L. kaempferi). He was awarded an honorary doctorate from the University of Bern in 1902.
As a climber and surveyor he made around 30 first ascents in the Alps, mostly in the Bernina Range, the Albula Alps, and the Upper and Lower Engadin. In 1846 he made the first ascent of Piz Kesch on 7 September, and in the same year he made the first ascent of Piz Languard, Piz Surlej, Piz Aguagliols, Piz d'Esen and Piz Lischana. In 1848 he made the first ascent of Piz Quattervals.