Using daily precipitation data from a network of weather stations across mainland Thailand, we apply a two-phase linear regression model to objectively determine the onset, withdrawal, and length of the summer monsoon season for the years 1951–2005. Our onset metric compares favorably with an independent determination of onset. Both onset and withdrawal are associated with expected wind and geopotential height anomalies in the lower atmosphere. Comparisons between stations show no coherent spatial variability in either onset or withdrawal, and trends at each station are small and statistically insignificant at the p < 0.05 level. When averaged across all stations, onset, withdrawal, and season length all show significant correlations with sea surface temperatures (SST) in the Indian ocean, tropical Pacific, and in the North Pacific regions with relatively well understood connections to monsoon variability. Additionally, there are also significant correlations with SSTs in the South Atlantic and North Atlantic, teleconnections that have been previously suggested but remain controversial. Compared to other methods for deriving the onset and withdrawal of the monsoon, our method provides one of the most objective techniques available using data readily available from most meteorological stations.