It is endemic to California, where it grows in wet granite rock habitat covered in thin soil layers, such as vernal pools and seeps.

[2] 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.

[3] Juncus triformis is a small annual herb forming dense clumps of hair-thin red stems up to about 16 centimeters high.

The inflorescence is made up of one to eight tiny flowers atop each stem.

The flowers have a few greenish to bright red segments no more than 4 or 5 millimeters long.