Poecilia mexicana, commonly known as the shortfin molly or Atlantic molly, is a species of poeciliid fish native to fresh and brackish water in Mexico and Guatemala.

[2] The fish lives in tropical freshwater and brackish water habitats.

[2] It is an invasive species in the Muddy River of Nevada, USA.

It is occurring between cave and surface populations as well as between habitats of varying sulfur concentrations.

[2][6] Poecilia refers to the Greek word poikilos, which means "with a lot of colours".