Plant phenotype is a key driver of plant–plant and plant–environment interactions and is central to predicting ecosystem responses in mixed cropping systems. Growing oilseed rape (OSR, Brassica napus) with legumes, as an agroecological strategy, can improve crop performance and sustainability. However, the mechanisms driving phenotypic plasticity in companion planting systems (CPS) and the underlying processes responsible for improved performance remain unclear. Using an interdisciplinary experimental approach integrating methods and concepts from ecophysiology, chemical ecology, and agroecology, we linked OSR phenotypic responses to agronomic outcomes. We compared three OSR varieties grown either as monocrops or with faba bean (FB, Vicia faba) companion plants under controlled glasshouse conditions and across two field seasons. We quantified morphological development, phenology, and glucosinolate-based defense traits, while monitoring crop microclimate and insect pests. Growing OSR with FB companion plants consistently reduced OSR leaf development under both greenhouse and field conditions, mainly because of reduced photosynthetically active radiation under the FB canopy, highlighting light-driven plasticity in early vegetative traits. During the reproductive phase, the release from light competition following FB frost damage allowed OSR to compensate for this initial delay through enhanced stem elongation and branching, contributing to yield maintenance or improvement. In parallel, CPS can contribute to insect pest regulation, potentially through modifications of glucosinolate profiles² combined with the visual and physical disruption provided by companion plants³. However, glucosinolate concentrations and profiles were strongly influenced by environmental conditions and genotype, indicating context-dependent modulation of plant defense traits. Overall, our results demonstrate that, beyond direct pest suppression, the benefits of CPS can also arise from morphological and phenological adjustments that improve OSR adaptation to interspecific interactions.