Euphorbia lathyris is an erect biennial (occasionally annual) plant growing up to 1.5 m tall, with a glaucous blue-green stem. The leaves are arranged in decussate opposite pairs, and are lanceolate, 5–15 cm long and 1-2.5 cm broad, glaucous blue-green with a waxy texture and pale greenish-white midrib and veins. The flowers are green to yellow-green, 4 mm diameter, with no petals. The seeds are green ripening brown or grey, produced in globular clusters 13–17 mm diameter of three seeds compressed together.

Herbs, annual, erect, up to 1(-1.5) m tall. Rootstock a simple taproot, slightly swollen in seedlings, up to more than 20 cm × 3-7 mm thick; lateral roots thin and branched. Stem single, gray-green, smooth and glabrous. Leaves opposite, decussate; stipules absent; petiole absent; leaf blade linear-lanceolate, 6-15(-20) × 0.4-2.5 cm, glabrous, base ± clasping stem, margin entire, apex acuminate or acute; midrib prominent adaxially, lateral veins inconspicuous. Inflorescence a terminal pseudumbel.