Coastal ecosystems face a multitude of pressures including plastic pollution and increased flood risk due to sea level rise and the frequency and severity of storms. Experiments seldom examine multiple stressors such as these, but here we quantified the effect of microplastics (polyethylene terephthalate (PET): a durable plastic and polybutylene adipate terephthalate (PBAT): a biodegradable polymer), in combination with simulated seawater inundation on the coastal species Plantago coronopus. After 35-days exposure to plastic (0.02 g.Kg−1, <300 μm diameter), P. coronopus were flooded to pot height with artificial seawater for 72-h, drained and grown for a further 24-days. Plant mortality, necrosis and photosynthetic efficiency (Fv/Fm) were recorded throughout, with root:shoot biomass and scape production (flower stalks) quantified at harvest. There were significant interactions between microplastics and seawater on the root:shoot ratio; a measure of resource allocation. The allocation to belowground biomass increased significantly under the PET + inundation treatment compared to the PBAT + inundation and the no plastic + inundation treatments, with potential consequences on the capture of water, nutrients and sunlight, which can affect plant performance. Plant necrosis significantly increased, and Fv/Fm declined as a result of seawater inundation. While not significant, plant Fv/Fm responses were influenced by microplastics (17% and 7% reduction in PBAT and PET exposure respectively compared to the no plastic control). Plants mediated this stress response with no discernible treatment-specific effects detected in Fv/Fm 14-days after seawater introduction. Plastic exposure significantly influenced potential reproductive output, with lower average scape numbers across PBAT treatments, but higher in PET treatments. This study highlights the complex interactions and potential for microplastics to present an elevated risk when in combination with additional stressors like seawater flooding; establishing the threat presented to ecosystem resilience in a changing world is a priority.