Juncus mertensianus is a species of rush known by the common name Mertens' rush. It is native to much of western North America from Alaska to Saskatchewan to New Mexico, where it grows in wet mountainous areas such as riverbanks and alpine meadows. This is a clumping perennial herb growing from a vertical rhizome. Its smooth, flat stems grow to a maximum height near 40 centimeters. Its few leaves are located at the base and also along the stem.