Perennials or subshrubs. Tri-chomes of leaves primarily 2-rayed, sometimes mixed with 3-rayed ones. Stems ascending to sprawling, proximal branches terminating in sterile rosettes, (woody at base), 0.5-3 dm. Basal leaves: blade linear to linear-oblanceolate, (2-)3.7-9 cm × 2-5(-10) mm, base attenuate, margins usually entire, rarely sparsely denticulate, apex acute. Cauline leaves (distal) sessile; similar to basal. Racemes considerably elongated in fruit. Fruiting pedicels widely spreading, slender, narrower than fruit, (8-)10-22 mm. Flowers: sepals oblong to linear-oblong, 6-10 mm, lateral pair slightly saccate basally; petals yellow, broadly obovate to suborbicular, (11-)14-20(-22) × (3-)4-11.5 mm, claw 7-10 mm, apex rounded; median filaments 7-10 mm; anthers linear, 2-3 mm. Fruits ascending to spreading, narrowly linear, straight or only slightly curved inward, not torulose, 2-5(-7) cm × 2-3 mm, angustiseptate, not striped; valves with prominent midvein, pubescent outside, trichomes 2-4-rayed, glabrous inside; ovules 36-50 per ovary; style cylindrical or slightly flattened, stout, 1-4 mm, sparsely pubescent; stigma slightly 2-lobed, lobes as long as wide. Seeds oblong, 1-2 × 1.2-2 mm; not winged. 2n = 36.

Flowering Mar-May. Sandy areas along coastal bluffs, coastal scrub; 0-300 m; Calif.

Erysimum insulare is restricted to the northern Channel Islands (San Miguel and Santa Rosa islands) of western Ventura County.