The genus Justicia (Acanthaceae) is represented by 12 species in the Mexican portion of the Yucatan Peninsula (MYP), one of which, Justicia cardinalis, is herein proposed as new to science. The new species is known from sub-xerophytic or seasonally dry forests over calcareous, rocky soils along the northern and northeastern regions in the MYP. This taxon has been confused with J. carthagenensis for more than 100 yr, but there are remarkable differences between the two taxa. The new species has linear to narrowly obovate (vs. oblong-spathulate to spathulate) floral bracts and narrower leaves. The corolla is red with white markings in the lower lip (vs. purple with white markings on the central lower lip and the upper lobes), and the corolla tube is as long as the lobes, whereas it is shorter than the lobes in J. carthagenensis. We provide relevant visual resources to aid in the identification of the new species. The conservation status is preliminarily assessed as Least Concern (LC) using the B criteria of the IUCN. Furthermore, we provide a brief synopsis, a distribution map, and a key to the species of Justicia from the MYP (as well as photographs of most of them). Finally, to be able to apply the names in an unambiguous manner, we designate a lectotype and an epitype for J. carthagenensis, and a lectotype for J. spicigera.