This paper presents a mathematical model developed in TRNSYS to simulate the performance of a solar heating system for an outdoor swimming pool in regions with a warm climate. For this purpose, a new type for TRNSYS was developed. The model was validated using experimental data collected from an outdoor 53.8 m(3) swimming pool in Cuernavaca, Morelos, Mexico. The data used to confirm the model's components and the full model were gathered from March 2016 to June 2017. This pool is located in a hotel surrounded by trees and vegetation that cause shading on the pool's surface throughout the day, thus reducing the heat gain from direct solar radiation. A shading factor equation for the shading over the pool was developed, introduced, and validated in the model to consider variations in pool temperature. For evaluating the evaporative losses, six empirical correlations obtained from the literature were tested. The model margin error was estimated at less than +/- 2% (an average of +/- 0.41%) with a temperature differential of less than +/- 0.5 degrees C (an average of +/- 0.12 degrees C, root of the mean quadratic error (RMSE) = 0.148 degrees C, mean bias error (MBE) = -0.058 degrees C, and coefficient of determination (R-2) = 0.9723) between the measured and simulated pool temperatures. Therefore, the model adequately reproduced the pool's temperature under different working conditions, and can be a valuable tool for generating a technical and economic analysis of solar heating systems in outdoor pools for regions with similar climatic conditions.