Open-air swimming pools in Mediterranean climate regions are heated by direct solar radiation with no auxiliary heating systems. In order to extend the swimming season or improve comfort conditions, solar collectors or pool coverings may be used. In this paper, another approach was followed through the use of phase change materials (PCM). Two methods of introducing the PCM were considered: (1) encapsulated in the sidewalls and bottom of the pool, and (2) use the PCM in an external heat exchanger. Heat is stored when water temperature is sufficiently high, while it is released when water temperature drops below comfort levels. A numerical model was built and validated for predicting the evolution of the water temperature, by taking into account meteorological data at three different locations in north-east of Spain. The simulations showed that using phase change materials provided some improvement of water conditions, especially when used in an external heat exchanger.