The aims of this research are to study vertically integrated moisture flux convergence (VIMC) over Southeast Asia and to analyse its relationship to rainfall over Thailand during the period 1999 to 2013. Data reanalysed by the National Oceanic and Atmospheric Administration (NOAA) during the period 1999 to 2013 are used in this study. The monthly mean rainfall data are taken from the Global Precipitation Climatology Project (GPCP). Vertically integrated moisture transport (VIMT) is calculated by vertically integrating moisture fluxes of the u and v components. The finite difference method is applied to the vertically integrated moisture flux divergence (VIMD). The results show that VIMD over the Indian Ocean is strong, and the moisture is directed from the Indian Ocean to Thailand by southwest winds that cause strong moisture convergence over Thailand during the rainy season, while moisture in the summer season is a strong divergence. Moisture increases from the South China Sea to Thailand during October to December, causing more moisture convergence over northern and northeastern Thailand. That the relationship between rainfall and VIMC averaged over Thailand from the years 1999 to 2013 is confirmed by large positive correlations. The average from the years 1999 to 2013 over the study area is confirmed by Thailand's rainfall pattern.