Plants to 1 cm. Stem leaves erect when dry, slightly flexuose to erect when moist, lanceolate, 2--3.2(--3.5) mm, with acute apex; margins entire or papillose-crenulate; distal laminal cells (7--)11--14(--16) µm wide; 1-stratose, basal laminal cells rectangular to almost quadrate, rarely nodose. Sexual condition goniautoicous or cladautoicous. Seta 0.4--0.6 mm. Capsule 2/3 emergent to shortly-exserted; urn cylindric, 8 narrow ribs entire length; stomates immersed; peristome double; endostome segments 16, filiforme, exostome teeth 16, erect when mature, reflexed when old. Calyptra conic-oblong, slightly plicate, with scattered short, thin hairs. Spores 16--18(--19) µm.

Trees; elevation unknown; Calif. Endemic.

This species is known only from type collections in the Santa Clara Mountains of Santa Clara County, California. Orthotrichum underwoodii has 8--16 lightly ribbed cylindric capsules with an erect exostome of 16 teeth and endostome of 16, long, linear segments. In peristome features it is similar to O. diaphanum, but that species has hair points. Perhaps O. underwoodii could be confused with O. tenellum, but the latter species has much more strongly ribbed capsules, reflexed exostome, endostome of 8, well-developed but short segments, and blunt, very stiff leaves with incurved tips.