The radiation of neotropical echimyid rodents resulted in the diversification of spiny rats, hutias, and the coypu, grouped into four major clades. Different echimyid lineages specialized for terrestrial, semifossorial, semiaquatic, and arboreal locomotion, and for living in bamboo thickets. Comparative phylogenetic methods were herein used for evaluating the relative effect of historical and ecological factors in shaping morphometric traits of external morphology (length of head and body, tail, ear, and hindfeet). Additionally, we investigated whether the adoption of different locomotory habits was associated with changes in their macroevolutionary dynamics. Our findings showed that variation of all traits was phylogenetically structured, although size was less structured than other traits. Tail, ear, and mainly hindfeet were allometrically correlated with head and body length, indicating the occurrence of evolutionary structural constraints. In addition to phylogenetic and allometric components, morphometric variation was strongly associated with locomotory specializations, except for body size. Disparification of body size and tail length took place gradually, fitting the Brownian motion model albeit with some punctual shifts in evolutionary rates. Some of these shifts were associated with changes in locomotory habits, notably with adoption of a semiaquatic habit by the large‐sized Myocastor lineage, which occurred with an accelerated rate of size evolution. Evolutionary changes in ear and hindfeet length were concentrated during early echimyid diversification, concomitantly with the emergence of most locomotory habits, with subsequent deceleration of evolutionary rates. These findings indicated a complex interaction between phylogenetic, structural, and ecological effects gradually shaping echimyid external morphology.