The Northern Pin Oak or Hill's Oak (Quercus ellipsoidalis) is an oak in the red oak section Quercus sect. Lobatae. It is mainly native to the northern midwest United States, and also in the southeast and southwest of Ontario, Canada. It occurs on dry, sandy, usually acidic soils. Although the name suggests an alliance to the Pin Oak Q. palustris, it has traditionally been thought to be closely related to the Scarlet Oak Q. coccinea, and was in fact included in that species by many botanists. However, recent work suggests that Hill's oak is more closely related to Black Oak Q. velutina and that there may be ongoing gene flow between those species (Hipp and Weber 2008). The morphological similarity between Q. ellipsoidalis and Q. coccinea remains a source of confusion, especially in northwestern Indiana and southern Cook County, Illinois.
It is a medium-sized deciduous tree growing to 20 m tall with an open, rounded crown. The leaves are glossy green, 7-13 cm long and 5-10 cm broad, lobed, with five or seven lobes, and deep sinuses between the lobes. Each lobe has 3-7 bristle-tipped teeth. The leaf is nearly hairless, except for small tufts of pale orange-brown down where the lobe veins join the central vein. The acorns tend to be ellipsoid (ellipse-shaped, from which its scientific name derives), though they tend to be highly variable and range to globose, 6-11 mm long and 10-19 mm broad, a third to a half covered in a deep cup, green maturing pale brown about 18 months after pollination; the kernel is very bitter. The inner surface of the acorn cap is glabrous (hairless) to sparsely or moderately pubescent, and the hairs if present tend to be kinky rather than straight.