Shrubs, rarely small trees , subevergreen, shrubs to 2 m, trees to 5 m or taller. Bark scaly on older branches and trunk. Twigs brownish or reddish, minutely puberulent, becoming glabrate and gray with age. Buds light or chestnut brown, ovate or globose, 2-3 × 1-2 mm. Leaves: petiole 2-5 mm. Leaf blade obovate or oblong, planar to moderately convex or undulate, 15-40 × 7-20(-40) mm, base cuneate or rounded, attenuate-decurrent along petiole, margins minutely cartilaginous, entire or with 1-5 irregular teeth on each side, secondary veins obscure, 1-5 on each side, apex blunt or rounded, occasionally subacute with mucronate tip; surfaces abaxially waxy, glandular, with scattered minute, flat, appressed, ± 8-rayed stellate hairs, not obscuring surface, adaxially green, glossy, glabrate or with minute, scattered, stellate hairs. Acorns paired or solitary in leaf axil, subsessile, rarely pedunculate in teratological forms; cup hemispheric to turbinate, to 15 mm deep × 20 mm wide, enclosing only 1/4-1/2 nut, scales moderately to heavily tuberculate, irregularly formed; nut light brown, acute-cylindric or fusiform, tapered, (15-)20-30 × (6-)9-15 m, apex acute, glabrate. Cotyledons distinct.

Flowering spring. Chaparral, oak woodlands, margins of grasslands, understory in closed-cone pine stands; of conservation concern; 0-300 m; Calif.

Quercus pacifica is endemic on three of the California Channel Islands: Santa Cruz, Santa Catalina, and Santa Rosa. It is not known from the mainland, but it bears a superficial similarity to some of the tree forms that are putative hybrids between Q . engelmannii and Q . cornelius-mulleri in San Diego County. The latter populations, sometimes treated as Q . × acutidens , differ in having much greater variability in leaf shape; thicker, more leathery leaves; denser abaxial leaf vestiture; much smaller hairs, having more than 10 rays; and variable levels of connation of cotyledons (always distinct in Q . pacifica ). Quercus pacifica appears to be most closely related to Q . douglasii , whether by direct descent or by introgression with another species no longer extant on the islands.

Quercus × macdonaldii Greene (as a species) [= Quercus dumosa var. macdonaldii (Greene) Jepson] is a stabilized hybrid complex between Quercus pacifica and Q . lobata Née. The plants tend to be small to moderate trees with leaves that resemble those of Q . lobata ; the leaves are much more shallowly lobed and always less than two-thirds the distance from the margin to the midrib. Quercus × macdonaldii is known from Santa Rosa, Santa Cruz, and Santa Catalina islands.