Juncus triformis is an uncommon species of rush known by the common names Yosemite dwarf rush and long-styled dwarf rush. It is endemic to California, where it grows in wet granite rock habitat covered in thin soil layers, such as vernal pools and seeps. It is known from the Sierra Nevada, Central Valley, southern Cascade Range, and locations in the eastern Transverse Ranges and the Peninsular Ranges in Southern California.