Cosmos atrosanguineus is a herbaceous perennial plant growing to 40–60 cm tall, with a fleshy tuberous root.

The flowers are produced in a capitulum 3–4.5 cm diameter, dark red to maroon-dark brown, with a ring of six to ten (usually eight) broad ray florets and a center of disc florets; they have a light vanillin fragrance (like many chocolates), which becomes more noticeable towards the end of the day.

[4] In 2008, Oku, T.; Takahashi, H.; Yagi, F.; et al. analyzed the Chocolate Cosmos flower using PSID (plastid subtype identity) sequences in order to clarify the phylogenetic relationships of this plant.

Field work showed that it grew in the states of Guanajuato, Querétaro and San Luis Potosí.

As of 2018[update], C. atrosanguineus 'Hamcoec' (trade description Eclipse) has the largest flower heads, up to 5 cm in diameter.