Rhizomes superficial or very shallow, cordlike portion 1-2 dm × 2 mm, expanding to 10-15 mm diam., nodes with brown, scalelike leaves and few to no roots. Stems 1-2-branched, 3-3.2 dm. Leaves: basal with blade pale green, 3.2 dm × 1.5 cm, margins scarious basally, apex acute, proximal 2 semi- sheathing, blade 5 cm, scarious; cauline 1-2, sheathing branch and stem for ca. 1/2 their length, blade ca. 5 cm, semimembranous. Inflorescences 2-3-branched, each unit with single flower, all borne at approximately the same level; spathes 2-3 cm × 5 mm, subequal, scarious except basally and along midrib. Flowers: perianth white or blue tinged with deep violet lines; floral tube funnelform, 0.3 cm; sepals oblong-spatulate, 2.8 × 1 cm, apex deeply emarginate, signal an inconspicuous crest with low, yellow, undissected ridge; petals bluish white, oblanceolate-spatulate, base gradually attenuate into claw; ovary elliptical, 0.4-0.7 cm; style 1.8 cm, crests broadly obovate, 0.7 cm, margins erose; stigmas triangular-acuminate, margins entire; pedicel 0.4-1 cm, not lifting flower clear of spathes. Capsules depressed-globose, roundly triangular, 0.9-1.5 × 1.2 cm. Seeds pale brown, with whitish raphe, D-shaped, pitted. 2n = 28.

Flowering May. Open, wooded slopes in leafy soil with Douglas fir and dense shrubs; of conservation concern; Oreg.

Iris tenuis was originally placed in ser. Californicae, but R. C. Foster (1937) stated, 'It is with some hesitation that I leave it in association with them. The broad, pale green leaves are much like those of a giant I. cristata.' F. H. Smith and Q. D. Clarkson (1956) said, 'It clearly does not belong in the subsection with the other members of the Californicae,' and Clarkson in a later treatment (1958) created a new subsection, the Oregonae, for it. L. W. Lenz (1959b) moved this species into subsect. Evansia (= sect. Lophiris), with which it shows many relationships.