Phenotypic plasticity of ecologically relevant traits relating to physiology, behavior, morphology, and life history is widespread and can affect the ecology and evolution of organisms. In streams, phenotypic plasticity in morphology and corresponding differences in performance have been documented for fishes and, to a lesser extent, for macroinvertebrates. Morphological plasticity in crayfish has not been addressed but, if present, has the potential to interact with behaviorally plastic traits to affect performance in high water velocities. Lake and stream invasions of rusty crayfish, Orconectes rusticus, provide excellent opportunities to examine morphological variation in habitats differing in water velocity. Spread of O. rusticus and displacement of resident species in lakes has been attributed to larger mean body and chelae sizes than for native species, but larger body and chelae sizes may not be advantageous in high-velocity environments. We examined populations of male O. rusticus (form I and II) from lakes, low-velocity streams (LVS ≈ 25 cm/s), and high-velocity streams (HVS ≈ 50 cm/s) to test the hypothesis that size and shape are affected by water velocity. We analyzed scanned images of chelae and body with geometric morphometrics. Body size differed significantly between HVS populations and LVS and lake populations, which did not differ in size or shape. Shape tended to differ between HVS populations and LVS and lake populations. Chelae of form I but not of form II crayfish were significantly smaller in HVS than in LVS and lake populations. Chelae (form I and II) in HVS populations were broader and stunted relative to chelae in LVS and lake populations. Chelae may help deflect water over the bodies. Morphological plasticity in crayfish adapted to HVS may alter invasion rate or success with subsequent effects on stream ecosystems.