Cellulose nanocrystals (CNCs) derived from biomass have unique properties, which have inspired their incorporation into a wide variety of materials. However, the number of highly stretchable elastomers that have been prepared with CNCs has been limited. Here, we report shear-aligned pseudonematic CNCs embedded in a poly(ethyl acrylate) elastomer, a homogeneous composite that exhibits reversible optical properties in response to mechanical stimuli. Due to the long-range anisotropy of CNCs, the relaxed composite shows vivid interference color as it is viewed between crossed or parallel polarizers. When the pseudonematic CNC elastomer is stretched parallel to the CNC alignment direction, the CNCs become further aligned and the birefringence of the materials increases. In contrast, when the composite is stretched perpendicular to the CNC alignment direction, the CNCs become more disordered and the birefringence decreases. The extent of the CNC reorientation when the composite was stretched was determined by calculation of the birefringence of the material and two-dimensional X-ray diffraction analysis. Furthermore, the aligned CNCs act as nanoreinforcement in the elastomer, which resulted in the pseudonematic CNC poly(ethyl acrylate) elastomer having a tensile modulus up to 120 times higher than that of pure poly(ethyl acrylate).