• Background and Aims Ecosystem-based flood defence including salt-marsh as a key component is increasinglyapplied worldwide due to its multifunctionality and cost-effectiveness. While numerous experiments haveexplored the wave-attenuation effects of salt-marsh plants critical to flood protection, little is known about thephysiological and biochemical responses of these species to continuous wave exposure. • Methods To address this knowledge gap, we developed a shallow-water wave simulator to expose individualSpartina alterniflora plants to waves in a greenhouse for 8 weeks. S. alterniflora individuals were partially submergedand experienced horizontal sinusoidal motion to mimic plant exposure to shallow water waves. A factorialexperiment was used to test the effects of three wave heights (4.1 cm, 5.5 cm and a no-wave control) and two waveperiods (2 s and 3 s) on the following key physiological and biochemical plant parameters: plant growth, antioxidantdefence and photosynthetic capacity. • Key Results Comparison of wave treatment and control groups supported our hypotheses that wave exposureleads to oxidative stress in plants and suppresses plant photosynthetic capacity and thereby growth. In response,the wave-exposed plants exhibited activated antioxidant enzymes. Comparison between the different wave treatmentgroups suggested the wave effects to be generally correlated positively with wave height and negativelywith wave period, i.e. waves with greater height and frequency imposed more stress on plants. In addition, waveexposedplants tended to allocate more biomass to their roots. Such allocation is favourable because it enhancesroot anchorage against the wave impact. • Conclusions Simulated wave exposure systems such as the one used here are an effective tool for studying theresponse of salt-marsh plants to long-term wave exposure, and so help inform ecosystem-based flood defence projectsin terms of plant selection, suitable transplantation locations and timing, etc. Given the projected variabilityof the global wave environment due to climate change, understanding plant response to long-term wave exposurehas important implications for salt-marsh conservation and its central role in natural flood defence.@en