Phenotypic plasticity may be adaptive if the phenotype expressed in a focal environment performs better there relative to alternative phenotypes. Plasticity in morphology may particularly benefit modular organisms that must tolerate environmental change with limited mobility, yet this hypothesis has rarely been evaluated for the modular inhabitants of subtidal marine environments. We test the hypothesis for Asparagopsis armata, a clonal red seaweed whose growth-form plasticity across light environments is consistent with the concept of foraging behaviour in clonal plants. We manipulated the light intensity to obtain clonal replicates of compact, densely branched ( phalanx ) phenotypes and elongate, sparsely branched ( guerrilla ) phenotypes, which we reciprocally transplanted between inductive light environments to explore the performance consequences of a poor phenotype-environment match. Consistent with the hypothesis of adaptive plasticity, we found that performance (as relative growth rate) depended significantly on the interaction between growth form and environment. Each growth form performed better in its inductive environment than the alternative form, implying that this type of plasticity, thought to be adaptive for clonal plants, may also benefit photoautotrophs in marine environments. Given the prevalence and diversity of modular phyla in such systems, they offer a relatively unexplored opportunity to broaden our understanding of the evolutionary ecology of phenotypic plasticity.