Poecilia mexicana, commonly known as the Atlantic molly, is a species of poeciliid fish native to the Atlantic slope of Middle America. The species is highly variable in coloration, shape, and size. Atlantic mollies occur in fresh and brackish waters, shallow and slow-flowing or standing. They feed chiefly on algae and detritus. Two populations are found in caves and represent some of the best-studied cavefish. It hybridizes with other Poecilia species in its range; one such cross has resulted in the unisexual P. formosa.

Poecilia mexicana was described in 1863 by Steindachner. Rosen and Bailey reduced P. mexicana and other shortfin mollies to synonyms of P. sphenops, a similar species which occurs throughout much of the range of P. mexicana. In 1971, Schultz and Miller restored P. butleri to species rank on the grounds of partial reproductive and geographic isolation. Two years later, Menzel and Darnell resurrected P. mexicana from synonymy of P. sphenops, noting that it differs from P. sphenops in ways similar to P. butleri. The key difference is dental: P. butleri and P. mexicana have inner jaw teeth with a single point (unicuspid), whereas P. sphenops has inner jaw teeth with three points (tricuspid). Menzel and Darnell recognized a subspecies, P. mexicana limantouri.

Poecilia mexicana is a slim, laterally flattened molly with a wide array of forms. The maximum known standard length (SL) is 95 mm. Mature males occur in multiple size classes, ranging from 18 mm to more than 70 mm. The head is flat on top and triangular in side view, narrowing to a pointed snout. The mouth opens at the front, is straight, and can be pushed forward. Both jaws carry narrow bands of teeth; the outer teeth are long, fine, and curved backward, forming a brush-like edge. Eyes are large, about one third of head length, and set high on the head.

There are clear differences between the sexes. Males are more slender, while females are deeper-bodied. Females have a broader back and a strongly rounded belly, whereas males are less curved. The tail base is relatively deep, especially in females. The dorsal fin begins slightly before the middle of the body and is rounded. Pectoral fins are large and nearly as long as the head; pelvic fins are shorter. The anal fin is tall and somewhat pointed, and the tail fin is broadly rounded and slightly longer than the head, with scales covering much of its base.

Scales are fairly large and cover most of the head except the jaws. Along the side of the body there are about 28 scales, with 10–11 between the dorsal and pelvic fins and about eight around the tail base. Most scales show a small central pore linked to the lateral line sensory system. The upper body is brown, gradually changing to golden yellow below. Each scale is darker at the base and along its rear edge. Dark spots on the sides vary: many males lack them, but some individuals show a row of spots, while females may have several rows running lengthwise along the body. The dorsal fin always has several rows of dark spots; the pelvic and anal fins are golden yellow, and the pectoral fins are dull yellow to brownish.

The two subspecies differ mainly in overall body form and fin proportions. P. m. limantouri has a more slender, almost cylindrical body, with a relatively short dorsal fin and a narrow caudal fin. In contrast, P. m. mexicana is deeper-bodied and somewhat laterally compressed, with a noticeably longer dorsal fin and a broader caudal fin. In areas where their ranges meet, individuals often show intermediate features.

Poecilia mexicana occupies much of the Atlantic slope of Middle America, extending from the lower basin of the Río Bravo (including the Álamo and San Juan rivers) south through Central America to Costa Rica, where it reaches the Río Matina. Offshore populations occur on the Bay Islands of Honduras. Within Mexico it has been recorded from Campeche, Chiapas, Hidalgo, Nuevo León, Oaxaca, Puebla, Querétaro, Quintana Roo, San Luis Potosí, Tabasco, Tamaulipas, Veracruz, and Yucatán. On the Pacific slope it is known only from the upper Río Choluteca basin in Honduras. The type locality was given as Orizaba, but the specimens almost certainly came from farther east, probably the Río Blanco or one of its tributaries. The species has also been introduced into the Río Lerma basin on the Pacific slope of Mexico.

Poecilia mexicana inhabits a wide range of lowland aquatic environments, including coastal lagoons, estuaries, ponds, and rivers, and in Mexico it also penetrates upland streams to at least 600 m elevation. The species tolerates fresh, brackish, and saltwater, having been recorded at salinities up to 32.4 ppt. It typically occurs in shallow water, often less than 1 m deep, occupying pools, riffles, or still habitats with little or no current. Substrates vary widely and include rock, mud, silt, sand, and rubble, though the fish is especially common over rocky bottoms coated with dense films of filamentous algae, diatoms, protozoans, and decomposing plant material. During the rainy season, dense aggregations often disperse into temporary waters such as roadside ditches, sometimes several kilometers from permanent habitats. Water clarity ranges from clear to highly turbid, and vegetation may be dense, particularly in oxbows of lowland rivers. Its broad ecological tolerance makes it a common fish along the Atlantic slope.