A new morning glory from the northern-most reaches of tropical forests in North America, Merremia cielensis , is described and illustrated. This narrow endemic from Tamaulipas, Mexico, shares close relationships with a complex of yellow-flowered merremias that produce indehiscent capsules with translucent pericarps. Merremia cielensis can be readily distinguished from all other Mexican merremias on the basis of cordate leaves, a ruddy, accrescent calyx during fruition, and a four-seeded capsule.