Daily rainfall records for 50 years (1961 to 2010) were used to determine the start and end of rainy seasons in the southwestern region of Sri Lanka. The study was based on data records obtained from two weather stations in Colombo and Galle situated in the western and southern coastal belts of Sri Lanka, respectively, that have two growing seasons, Yala and Maha. The start and end dates were determined by using a method that is based on cumulative rainfall measured during a season. Substantial interannual variability is seen in start and end of rainy seasons in this study. For the Yala season, the mean start and end dates are in the standard weeks 12.0 ± 2.3 and 24.1 ± 2.0 (March 24 to June 20), respectively. For the Maha season, the mean start and end dates are in the standard weeks 35.4 ± 2.0 and 49.6 ± 2.0 (September 5 to December 15), respectively. The mean durations of the rainy season for Yala and Maha are 12.1 ± 2.6 weeks and 14.2 ± 2.3 weeks respectively. The start date and the duration of the rainy season for both Yala and Maha are correlated, leading to early start dates resulting in longer rainy seasons. It can be concluded from this study that it is possible to use daily rainfall records to determine the mean start and end dates of rainy seasons in the southwestern region of Sri Lanka.