Diving beetles in the genus Platynectes (s. str.) in South America are relatively large and charismatically colored, yet relatively rare in museum collections. Recent fieldwork in northern and central South America has resulted in the collection of hundreds of new specimens that substantially expand our distributional knowledge of the genus. Here, we provide new distributional records for Platynectes decemnotatus (Aubé, 1838), P. garciai Gustafson, Short & Miller, 2016, P. submaculatus (Laporte, 1835), and P. tafelbergensis Gustafson, Short & Miller, 2016. In addition, two new species were identified and described herein: Platynectes bicolor sp. nov. of the P. ornatifrons species group from Brazil (Goiás and Mato Grosso do Sul), and P. colombicus sp. nov. of the P. submaculatus species group from the Guiana Shield region of eastern Colombia. These new records suggest that Platynectes (s. str.) is likely widespread across the Brazilian and Guiana Shield regions of the continent, and that some individual species have substantial range sizes of nearly 2000 km. An updated key to the species groups of Neotropical Platynectes (s. str.) and to the species of P. submaculatus species group are provided.