A worldwide increase in flooding due to climate change and population growth in exposed areas is expected, especially in coastal areas; therefore, nature-based solutions (NBS) for risk reduction are necessary to increase the resilience of cities, particularly in developing countries, which usually lack large budgets for structural measures but have natural areas such as wetlands that can be used as NBS. The flood mitigation ecosystem service of a coastal wetland in central Chile was analyzed. Using hydrological and topo-bathymetric data, two flood hazard scenarios were modeled: (i) S1 current and (ii) S2 projected, which was established based on land-use planning instruments and urban projects developed since 1954. Flood hazard maps for different return periods were obtained and indicators related to the mitigation potential of the wetland were calculated. It was proven that urban project development has intensified since 2000, mainly in the form of real estate development, with an increase in occupation of 50%, and the wetland area is projected to be further reduced by around one third, decreasing potential flood mitigation. Thus, for an extreme return period, in this case 500 years, the water volume stored by the wetland would decrease by more than 38% and the flooded area of the wetland by 30%, increasing flooding and vulnerability of the urban area, with various repercussions for surrounding neighborhoods and infrastructure. The number of people and homes affected would increase by around 6% and 8%, respectively, such that the affected land value would reach an additional US$55 million, which would be very detrimental in a city that has seen its natural spaces encroached upon by gray infrastructure. This research reaffirms the need to support the restoration and conservation of coastal wetlands under pressure from urban development in an area with a lack of green infrastructure planning.