The existence of at least two biological species of finless porpoises (Neophocaena phocaenoides and N. asiaeorientalis) was accepted only recently. Given the vulnerability of finless porpoises to human activities throughout their distribution, it is crucial that the two species be distinguished during field studies to better understand their biology and conservation status. Because finless porpoises are notoriously difficult to detect and to photograph at sea owing to their intrinsic cryptic nature (i.e., small size, no dorsal fin, brief surface period, avoidance of boats), differentiating between the two species in situ has often been neglected and attempts have usually been met with skepticism. Moreover, a large area (Taiwan Strait and adjacent waters) exists where the two species are sympatric, which further complicates the task of species identification. In this study, we demonstrated that the two species can be differentiated at sea by direct observation and examination of photographs. By comparing photographs of living and dead specimens of both species, we describe useful diagnostic field characters for species identification of free-ranging finless porpoises. Differentiating these species through direct observation was possible but not trivial and the most confident and convincing identifications were those supported by quality images.