Thuja plicata is a large to very large tree, ranging up to 65 to 70 m (213 to 230 ft) tall and 3 to 7 m (9.8 to 23.0 ft) in trunk diameter. Trees growing in the open may have a crown that reaches the ground, whereas trees densely spaced together will exhibit a crown only at the top, where light can reach the leaves. It is long-lived; some individuals can live well over a thousand years, with the oldest verified being 1460 years.
The foliage forms flat sprays with scale-like leaves in opposite pairs, with successive pairs at 90 degrees to each other. The foliage sprays are green above and green marked with whitish stomatal bands below; they are strongly aromatic, with a scent reminiscent of pineapple when crushed. The individual leaves are 1 to 4 mm (0.039 to 0.157 in) long and 1 to 2 mm (0.039 to 0.079 in) broad on most foliage sprays, but up to 12 mm (0.47 in) long on strong-growing lead shoots.
The cones are slender, 10 to 18 mm (0.39 to 0.71 in) long, and 4 to 5 mm (0.16 to 0.20 in) broad, with 8 to 12 (rarely 14) thin, overlapping scales. They are green to yellow-green, ripening brown in fall about six months after pollination, and open at maturity to shed the seeds. The seeds are 4 to 5 mm long and 1 mm (0.039 in) broad, with a narrow papery wing down each side. The pollen cones are 3 to 4 mm (0.12 to 0.16 in) long, red or purple at first, and shed yellow pollen in spring.