It is a herbaceous rhizomatous perennial plant growing 30–100 cm tall, with variably hairless to hairy stems and foliage, and a wide-spreading fleshy underground rhizome. The leaves are 5–9 cm long and 1.5–3 cm broad, with a serrated margin. The stem is square-shaped, a trademark of the mint family of herbs. Spearmint produces flowers in slender spikes, each flower pink or white, 2.5–3 mm long and broad.
Hybrids involving spearmint include Mentha × piperita (peppermint; hybrid with Mentha aquatica), Mentha × gracilis (ginger mint, syn. M. cardiaca; hybrid with Mentha arvensis), and Mentha × villosa (large apple mint, hybrid with Mentha suaveolens).
The name 'spear' mint derives from the pointed leaf tips.