Pin oak is a medium-sized deciduous tree growing to 18–22 metres (59–72 feet) tall, with a trunk up to 1 m (3+1⁄2 ft) in diameter. It has an 8–14 m (26–46 ft) spread. A 10-year-old tree grown in full sun will be about 8 m (26 ft) tall. Young trees have a straight, columnar trunk with smooth bark and a pyramidal canopy.
By the time the tree is 40 years old, it develops more rough bark with a loose, spreading canopy. This canopy is considered one of the most distinctive features of the pin oak: the upper branches point upwards, the middle branches are at right angles to the trunk, and the lower branches droop downwards.
The leaves are 5–16 centimetres (2–6+1⁄4 inches) long and 5–12 cm (2–4+3⁄4 in) broad, lobed, with five or seven lobes. Each lobe has five to seven bristle-tipped teeth. The sinuses are typically U-shaped and extremely deep cut. In fact, roughly the same amount of sinus area exists as actual leaf area. The leaf is mostly hairless, except for a very characteristic tuft of pale orange-brown down on the lower surface where each lobe vein joins the central vein. Overall autumn leaf coloration is generally bronze, though individual leaves may be red for a time, and is not considered particularly distinctive. The acorns, borne in a shallow, thin cap, are hemispherical, 10–16 millimetres (13⁄32–5⁄8 in) long and 9–15 mm (11⁄32–19⁄32 in) broad, green maturing pale brown about 18 months after pollination. Unless processed using traditional methods, the acorn is unpalatable because the kernel is very bitter.
In its natural environment pin oak is a relatively short-lived, fast-growing pioneer or riparian species with a lifespan of approximately 120 years against many oaks which can live several centuries. Despite this there are many examples of pin oak that exceed this lifespan. It develops a shallow, fibrous root system, unlike many oaks, which have a strong, deep taproot when young.
A characteristic shared by a few other oak species, and also some beeches and hornbeams, is the retention of leaves through the winter on juvenile trees, a natural phenomenon referred to as marcescence. Young trees under 6 m (20 ft) are often covered with leaves year-round, though the leaves die in the fall, remaining attached to the shoots until the new leaves appear in the spring. As with many other oak species, dead pin oak branches stay on the tree for many years.
Like all oaks, flowering and leaf-out occur in late spring when all frost danger has passed. The flowers are monoecious catkins which, being self-incompatible, require the presence of another oak for pollination. Any species in the red oak group can serve as a pollinator, but in pin oak's natural range, this will usually be northern red oak or scarlet oak. Interspecies hybridization occurs freely. The acorns require two growing seasons to develop.