Water oak is a medium to large tree with a tall straight trunk, a rounded, symmetrical crown, and ascending branches.
Leaves are alternate, simple, 2–4 inches long, broadest near the tip, fan-shaped, tapering to a long, narrow base; margin varies, sometimes entire with a rounded tip, often slightly wavy, usually with 3 broad lobes at the tip. Lobes sometimes with bristle tips. Leaf stalk less than ¼ inch long. Leaves turn yellow in fall; brown leaves persist into winter.
Bark is grayish-black, smooth, and tight on young trees, becoming shallowly grooved with wide, flat, scaly ridges with age.
Twigs are slender, reddish-gray, smooth.
Flowers April–May, in catkins.
Fruits September–October, acorns solitary or paired; brown, broadest at base and broadly rounded at the tip, about ½ inch long, cup covering to ½ of the nut, cup shallow, saucer-shaped, thin, reddish-brown; scales small, thin, flattened hairy; seeds bitter; ripening in autumn of the second year.