Ultrastructural descriptions of spermatozoa are a useful tool to reveal bivalve phylogeny. Several Donax species living off Latin America show strong morphological variability that often generates discussion on the validity of the species identity. This study is the first attempt to describe the structure of spermatozoa of Donacidae from the southwestern Atlantic Ocean. Donax hanleyanus (Philippi, 1842) is a free-spawning marine bivalve inhabiting exposed intertidal sandy beaches and has a geographic distribution along the Atlantic coast of South America, from Rio de Janeiro, Brazil (22°51S) to Mar del Plata, Argentina (38°20S). Transmission electronic microscope (TEM) and scanning electron microscopy (SEM) confirmed that euspermatozoa of D. hanleyanus can be classified as aquasperm type common for many bivalves. The head is composed of an spheroidal nucleus capped by a conical acrosome vesicle showing an anterior extension (total length 2 µm). The mid-piece region consists of an axoneme with a short basal fossa containing the centriolar complex and an initial portion of a 9+2 axoneme, sheathed by 4-5 spherical mitochondria. The single flagellum shows the typical 9+2 microtubule structure. Specifications useful for taxonomy are highlighted.