Features an oval and slightly flattened carapace that reaches an average length of 20 to 30 centimeters, with exceptional specimens growing up to 35 centimeters. The coloration of the carapace varies from dark brown to black, showcasing a subtle vertebral keel that tends to wear down as the individual ages. The plastron exhibits a yellowish or cream-colored base ornamented with irregular dark blotches that fade in older adults. Its head, broad and flat, is highly distinctive due to a unique cephalic pattern of light-colored, typically bright yellow stripes that form a horseshoe-like shape on the dorsal side of the neck and behind the eyes. Two small chin barbels are present on the lower jaw, serving a sensory function. The webbed limbs of this representative of the genus Phrynops are highly adapted for strong swimming, equipped with robust claws that allow it to cling firmly to rough surfaces underwater.

Geographical distribution: Occurs exclusively in temperate and subtropical regions of central-eastern South America. Its range encompasses southern Brazil (specifically the states of Rio Grande do Sul, Santa Catarina, and Paraná), northeastern Argentina (mainly in the provinces of Misiones and Corrientes), and northern and eastern Uruguay. It is closely associated with the river systems of the Uruguay, Paraná, and Iguazú basins, maintaining localized but well-adapted populations along these rugged, fast-flowing tributaries.

Habitat: Specializes in fast-flowing lotic environments, being considered a highly rheophilic organism. It prefers clear, highly oxygenated rivers and streams flowing over beds of stones, basalt, or gravel, which are characterized by rapids, waterfalls, and riffles. It is frequently found in areas where the riparian forest remains dense and well-preserved, as this canopy provides shade, protection from predators, and a steady input of organic matter that fuels the aquatic food web. It generally avoids slow-moving, turbid water bodies with muddy substrates.

Feeding: Exhibits a strictly carnivorous diet, with a strong preference for aquatic macroinvertebrates and malacophagy. Its diet primarily consists of aquatic snails, bivalves, dragonfly nymphs, caddisfly larvae, and other benthic insects found attached to river stones. It complements this diet by opportunistically preying on small fish, freshwater crabs, and occasionally tadpoles. Its powerful jaws, equipped with sharp edges, are evolutionarily optimized for crushing the calcareous shells of the mollusks that form the foundation of its diet.

Behavior: Displays primarily diurnal habits, dedicating most of the peak daylight hours to thermoregulation. It is commonly observed basking on emergent rocks in the middle of swift currents, maintaining a highly alert state that allows it to plunge rapidly into the water at the slightest sign of danger. It is an excellent swimmer, easily navigating turbulent currents thanks to its hydrodynamic body shape. Its temperament is solitary and elusive; interactions between conspecifics of Phrynops williamsi are mostly limited to the breeding season or competing for the best basking spots.

Reproduction: Begins its reproductive cycle during the spring months as water temperatures begin to rise. Gravid females actively search for elevated, dry locations along riverbanks, selecting deposits of sand, gravel-sand mixtures, or small sediment accumulations within rocky crevices to dig their nests, always choosing spots safe from seasonal flooding. The clutch typically consists of 5 to 15 spherical, hard-shelled eggs. The incubation period varies depending on environmental temperatures, lasting throughout the summer until the hatchlings emerge, which usually coincides with the rainy season when they head straight to the safety of shallow stream margins.

Conservation status: Listed globally as Data Deficient (DD) by the IUCN, due to a historic lack of consistent population monitoring. However, regional assessments suggest that populations of Phrynops williamsi face severe threats that place them in a vulnerable state. The primary threat is habitat loss and fragmentation caused by the construction of hydroelectric dams, which turn fast-flowing mountain rivers into deep, slow reservoirs, destroying the specific rapids they need to survive. Additionally, deforestation of riparian forests and siltation of rocky riverbeds severely impact the abundance of their prey.