Mangroves are widely distributed along the coastline of Vietnam, where they provide protection against sea waves caused by extreme weather. Impacts of climate change, together with population growth and economic development, are expected to exert pressure on these vulnerable systems. In this study the numerical wave-propagation model SWAN-VEG (Simulating Waves Nearshore–Vegetation) was used to simulate the possible impacts of climate change on the wave-dissipation capacity of different types of mangrove vegetation. Vegetation characteristics were assessed in planted plots (Rhizophora apiculata and a mix of R. mucronata, Sonneratia caseolaris, Avicennia alba, and Nypa fructicans) and in natural regenerated areas (A. alba and S. caseolaris) in Thanh Phu Natural Reserve, Mekong Delta, Vietnam; these assessments were used as model input. Different sea levels and mangrove vegetation characteristics were used to simulate the potential impacts of climate change. Planted plots with a cover of 70% reduced the height of incoming waves by 60%, compared with 40% for natural regenerated forest. Reducing the vegetation cover in planted plots from 70% to 50%, 35%, and 0% resulted in wave-height reductions of 51%, 42%, and -4%, respectively. A sea level rise (SLR) up to 0.96 m did not change the wave-dissipation potential of R. apiculata planted in the plots. However, an assumed decline in the width of vegetation from 1.5 km to 0.5 km, e.g. as a consequence of coastal erosion, reduced the height of incoming waves 21% (no SLR) and 29% (0.96 m SLR), as compared to 60% and 59%, respectively, without erosion.