This paper presents results from experiments in a large flume on wave and flow attenuation by a full-scale artificial Posidonia oceanica seagrass meadow in shallow water. Wave height and in-canopy wave-induced flows were reduced by the meadow under all tested regular and irregular wave conditions, and were affected by seagrass density, submergence and distance from the leading edge. The energy of irregular waves was reduced at all components of the spectra, but reduction was greater at the peak spectral frequency. Energy dissipation factors were largest for waves with small orbital amplitudes and at low wave Reynolds numbers. An empirical model, commonly applied to predict friction factors by rough beds, proved applicable to the P. oceanica bed. However at the lowest Reynolds numbers, under irregular waves, the data deviated significantly from the model. In addition, the wave-induced flow dissipation in the lower canopy increased with increasing wave orbital amplitude and increasing density of the mimics. The analysis of the wave-induced flow spectra confirm this trend: the reduction of flow was greatest at the longer period component of the spectra. Finally, we discuss the implications of these findings for sediment dynamics and the role of P. oceanica beds in protecting the shore from erosion. ► Full scale tests with Posidonia oceanica indicate that the seagrass reduces wave energy and wave-induced flows. ► Energy dissipation factors produced by the submerged canopy decay with wave orbital amplitude. ► Energy dissipation may be predicted by existing empirical formulae and canopy roughness may be estimated. ► In-canopy wave-induced flow reduction increases with increasing wave orbital amplitude and with increasing period of the flow spectra component. ► Effects of plant density, submergence ratios (hs/D) and distance from the leading edge were analysed.