Compared with other squamates, snakes have received relatively little ecomorphological investigation. We examined morphometric and meristic characters of vipers, in which both sidewinding locomotion and arboreality have evolved multiple times. We used phylogenetic comparative methods that account for intraspecific variation (measurement error models) to determine how morphology varied in relation to body size, sidewinding, arboreality, and precipitation. Some traits scaled isometrically; however, tail length was positively allometric and head dimensions were negatively allometric. Although we expected sidewinding specialists to have different body proportions and more vertebrae than non-sidewinding species, they did not differ significantly for any trait after correction for multiple comparisons. This result suggests that mechanisms enabling sidewinding involve musculoskeletal morphology and/or motor control, that viper morphology is inherently conducive to sidewinding (“preadapted”), or that behavior has evolved faster than morphology. With body size as a covariate, arboreal vipers had long tails, narrow bodies, and lateral compression, consistent with previous findings for other arboreal snakes, plus reduced posterior body tapering. Species from wetter environments tended to be larger, with longer heads and reduced anterior tapering. This study adds to the growing evidence that, despite superficial simplicity, snakes have evolved various morphological specializations in relation to behavior and ecology.