Morphological plasticity is a genotype-by-environment interaction that enables organisms to increase fitness across varying environments. Symbioses with diverse microbiota may aid in acclimating to this variation, but whether the associated bacteria community is phenotype-specific remains unstudied. Here we induce morphological plasticity in three species of sea urchins and measure changes in the associated bacterial community. While each host species had unique microbial communities, the expression of morphological plasticity resulted in the convergence for a phenotype-specific microbiome that was, in part, driven by differentially associating with α- and γ-proteobacteria. Furthermore, these results suggest that phenotype-specific signatures were the product of the environment, and are correlated with ingestive and digestive structures. By manipulating diet quantity over time, we also support that differentially associating with microbiota along a phenotypic continuum is bidirectional. Taken together, our data support the idea of a phenotype-specific microbial community and that phenotypic plasticity extends beyond a genotype-by-environment interaction.