Johann Jakob Weilenmann (24 January 1819 in St. Gallen – 8 June 1896 in St. Gallen) was a Swiss mountaineer and Alpine writer.
Weilenmann accomplished many first ascents in the Alps, amongst which are:
- 1859 - Muttler in the Samnaun Alps
- 1861 - Fluchthorn in the Silvretta, with Franz Pöll, a guide from Galtür
- 1862 - Ramolkogel and Weißer Kogel in the Ötztal Alps
- 1863 - Helsenhorn in the Lepontine Alps
- 1865 - Piz Buin in the Silvretta with Joseph Anton Specht and the guides Franz Pöll and Jakob Pfitschner
- 1865 - Crast' Agüzza in the Bernina Range with Specht, Pöll and Pfitschner
- 1865 - Mont Blanc de Cheilon in the Pennine Alps with Justin Felley