Historically, the South American locust, Schistocerca cancellata (Serville, 1838), has been considered the most serious agricultural pest in Argentina. An outbreak of a magnitude not recorded since 1954 started in 2015 through 2017 in northern Argentina and neighboring Paraguay and Bolivia. Schistocerca cancellata is widely considered as a true locust, with pronounced locust phase polyphenism, although the expression of its phenotypic plasticity has never been quantitatively tested under different density conditions. In this study, we explicitly quantified density-dependent reaction norms in behavior, coloration, and morphology in last instar nymphs of S. cancellata under isolated and crowded conditions. We also quantified density-dependent plasticity in adults (size) and in some life history traits. Our results showed that crowded nymphs were significantly more active and more attracted to congeners than isolated nymphs, and developed a much higher percentage of black pattern color. We also found that density had strong effects on body size and there was a sex-dependent pattern in both nymphs and adults, revealing that differences in size between males and females were less pronounced in crowded locusts. We have recorded for the isolated nymphs the presence of about 50% more hairs in the hind femora than in crowded nymphs. Finally, the mean duration of each nymphal instar and adult stage was significantly longer in isolated individuals. We have found strong resemblance with the desert locust, S. gregaria (Forskål, 1775) in several traits, and we conclude that S. cancellata exhibits an extreme form of density-dependent phenotypic plasticity in behavior, coloration, morphology, and life history traits.