Heterobranch sea slugs exhibit a wide range of body shapes, sizes and color patterns. The genus Placida (Sacoglossa) includes cryptic species, and in 2019 four species were recovered through molecular studies. Specimens of Placida collected in the Argentinian Sea (San Matias Gulf, Patagonia) between 2016 and 2019 are shown to belong to an undescribed species using molecular and morphological evidence. Placida sudamericana n. sp. is described using external, radular, penial stylet and egg mass characters. The specimens were always found associated with the green algae Codium fragile. Placida sudamericana is the only Placida species in the South Atlantic waters of South America and was recorded in a molluscan biodiversity hotspot.