Stems ascending to erect, 18–85 cm, glabrous, not glaucous. Leaves basal and cauline, ± leathery or not, glabrous, not glaucous; basal and proximal cauline 55–250 × 7–40 mm, blade lanceolate to elliptic, base tapered, margins entire, apex obtuse to acute; cauline 3–5 pairs, short-petiolate or sessile, 40–83 × 10–36 mm, blade ovate to lanceolate, base tapered to clasping, apex obtuse to acute. Thyrses continuous, cylindric, 4–35 cm, axis glabrous, verticillasters 4–11, cymes 1–6-flowered, 2 per node; proximal bracts ovate to lanceolate, 22–85 × 11–35 mm; peduncles and pedicels glabrous, peduncles to 20 mm, pedicels 1–6 mm. Flowers: calyx lobes ovate to lanceolate, (5.3–)6.5–8.5 × 1.6–3 mm, apex acuminate to caudate, glabrous; corolla blue to violet or purple, with reddish purple nectar guides, ventricose, 29–35 mm, glabrous externally, glabrous internally, tube 8–11 mm, throat gradually inflated, not constricted at orifice, 9–10 mm diam., slightly 2-ridged abaxially; stamens: longer pair reaching orifice, pollen sacs divergent to opposite, sigmoid, 2–2.7 mm, dehiscing incompletely, proximal 1/5–1/3 indehiscent, connective not splitting, sides sparsely hirsute, hairs white or tan, to 0.1 mm, especially near connective, sometimes merely scurfy near connective, sutures denticulate, teeth to 0.1 mm; staminode 16–20 mm, reaching orifice, 0.5–1 mm diam., tip straight, distal 4–6 mm ± uniformly sparsely pilose, hairs yellow, to 1 mm; style 20–24 mm. Capsules 10–13 × 8–10 mm.
Phenology: Flowering May–Jun.
Habitat: Rocky ridges, sandy to rocky slopes, coniferous forests.
Elevation: 1300–1800 m.
Penstemon pennellianus occurs primarily in the Blue Mountains of northeastern Oregon (Grant, Umatilla, Union, and Wallowa counties) and southeastern Washington (Asotin, Columbia, and Garfield counties).