A persistent inaccuracy in the spectral wind wave model SWAN is the poor prediction of the low-frequency part of the wind wave spectrum inside the tidal inlets of the Wadden Sea (Amelander Zeegat) and the Scheldt region. In the Eastern Wadden Sea, this inaccuracy is also found along the mainland coast. This concerns wind wave components generated in the offshore, that propagate through the tidal inlets without growing significantly further under the influence of the wind. Since an underestimation of these low-frequency components in the Hydraulic Boundary Conditions could result in an underprediction of the amount of run-up and overtopping over the sea defences, it is important to address the inaccuracy. Results in the literature suggest that the omission of diffraction in previous hindcast studies for the Wadden Sea may have contributed tot these inaccuracies. In this study, it is investigated wgether diffraction affects the penetration of low-frequency wind wave energy into the Dutch Wadden Sea and the Eastern Scheldt. To investigate the possible influence of diffraction, wave propagation patterns of three different types of models were studied for three geographical areas. The models are the ray-tracing model REFRAC, the spectral model SWAN, which was run with and without phase-decouple diffraction activated, and the mild-slope equation model PHAROS. The three domains studied were the Eastern Scheldt, the Amelander Zeegat and the Eastern Wadden Sea, for which inaccurate prediction of low-frequency components have been reported. The sensitivity of the results tot the offshore wave direction and imposed water levels was investigated. Subsequently, model results of hindcast cases were compared to observations. Wave diffraction, as included in the mild-slope model PHAROS, was not found to be an important process in the penetration of low-frequency wind waves into the regions studied. For the Eastern Scheldt study domain, this is also the case for the phase-decoupled formulation included in SWAN. It is therefore not recommended to improve the modelling of diffraction in SWAN in order to improve the prediction of low-frequency wind wave inshore.