Citrus × aurantiifolia is a citrus hybrid (C. micrantha x C. medica) with a spherical fruit, 2.5–5 cm (1–2 in) in diameter. The Key lime is usually picked while it is still green but it becomes yellow when ripe.

C. aurantiifolia is a shrubby tree, to 5 m (16 ft), with many thorns. Dwarf varieties exist that can be grown indoors during winter months and in colder climates. Its trunk, which rarely grows straight, has many branches, and they often originate quite far down on the trunk. The leaves are ovate, 2.5–9 cm (1–3 ¹⁄₂ in) long, resembling orange leaves (the scientific name aurantiifolia refers to this resemblance to the leaves of the orange, Citrus aurantium). The flowers are 2.5 cm (1 in) in diameter, are yellowish white with a light purple tinge on the margins. Flowers and fruit appear throughout the year, but are most abundant from May to September in the Northern Hemisphere.