Stems 5-25(-42) cm; base reddish or not, at least two proximal internodes long-pubescent. Leaves mostly on proximal 1/3 of stem; basal leaves 1-6 at anthesis; cauline leaves 2-4 at anthesis; petiole 0.5-7 cm. Leaf blade deltate to pentagonal, 1-2.5 × 1.5-4.5 cm, ± puberulent; ultimate lobes 3-15, width 3-17 mm (basal), 1-8 mm (cauline), widest at middle or in proximal 1/2. Inflorescences 2-10(-20)-flowered; pedicel 1-4(-6) cm, puberulent to glabrous; bracteoles (3-)7-11(-21) mm from flowers, green to blue, linear-lanceolate, 6-9(-12) mm, puberulent. Flowers: sepals bluish purple (somewhat faded on drying), puberulent, lateral sepals spreading, (11-)15-20(-24) × 6-11(-15) mm, spur straight, 0-30° above horizontal, 13-20 mm; lower petal blades ± covering stamens, 6-11 mm, cleft 2-4 mm; hairs on entire surface, densest on inner lobes, yellow or white. Fruits 9-20 mm, 3-4.5 times longer than wide, glabrous to puberulent. Seeds wing-margined; seed coat cells ± aggregate in small wavy ridges, cell surfaces ± roughened, without swollen, blunt hairs.

Delphinium decorum and D . menziesii have been confused. Delphinium menziesii usually has darker, colorfast (nonfading) sepals with less lavender, and a more northern distribution than D . decorum . It also has more finely dissected leaves than D . decorum subsp. decorum .

Often confused with Delphinium patens , D . decorum is sometimes circumscribed to include that species. The spreading, fading, blue-purple sepals, pubescent proximal portion of stems, and large lower petal blade of D . decorum , compared to the reflexed, colorfast, bluer sepals, proximally glabrous stems, and smaller lower petal blades of D . patens , adequately distinguish the two taxa.