ABSTRACT The high variability of natural environments poses significant challenges to photosynthetic organisms, which must adapt to constant fluctuations. Mechanisms such as acclimation and adaptation are essential for mitigating stress and ensuring survival. Rugulopteryx okamurae , an invasive alien seaweed recently introduced into Mediterranean and Atlantic waters, displays a remarkable ecological success, forming dense monospecific populations throughout diverse environments. This persistence suggests highly efficient acclimation mechanisms, possibly driven by seasonal physiological performance. The aim of this study was to evaluate the seasonal physiological response of R. okamurae to seasonal environmental fluctuations within a Posidonia oceanica meadow in the Alboran Sea, to better understand the photosynthetic acclimation strategies underlying its invasiveness. Photosynthesis–irradiance curves, pigment concentrations and elemental composition were analysed in individuals sampled bimonthly from July 2021 to July 2022 at a −10 m depth. Results showed significant seasonal variation in physiological parameters, highlighting a consistent acclimation capacity and robust photosynthetic performance. Fluctuations in pigment content and photosynthetic variables reflected the species' ability to optimise metabolic activity in response to environmental changes. Light compensation and saturation parameters further indicated strong photoadaptive capacity, enabling the species to thrive under both low and high light conditions. Its tolerance to a broad range of environmental factors, combined with mechanisms that prevent photoinhibition, may explain its persistence across seasons and wide depth ranges in the Alboran Sea. Although further research along depth gradients is needed, these findings underscore the role of physiological plasticity in the ecological success of R. okamurae and highlight the value of field‐based studies in understanding invasion processes of marine macrophytes.