Long‐term records of precipitation in Thailand are necessary to evaluate the robustness of the relationship between El Niño–Southern Oscillation (ENSO) and rainfall. This study presents a tree ring‐cellulose oxygen isotope (δ18O) record, based on Pinus merkusii, for the period 1804–1999 in Thailand. Response and spatial correlation analyses reveal that tree ring δ18O is significantly correlated with regional monsoon season (May–October) precipitation. Tree ring δ18O, which explains 50.1% of the variability in regional precipitation, was employed to reconstruct monsoon season rainfall back to 1804. Relatively wet periods occurred in 1809–1821, 1876–1882, 1897–1908, and 1944–1975, while the periods 1825–1850, 1913–1925, and 1979–1997 were relatively dry. During the periods 1854–1930 and 1970–1999, inter‐annual variability of precipitation was modulated by the ENSO. In contrast, the absence of this relationship between 1930 and 1970 might relate to the reduced variance of ENSO. Tree ring δ18O that is significantly correlated with monsoon season (May–October) precipitation is employed to reconstruct monsoon season rainfall in central Thailand during the period of 1804–1999. Inter‐annual variability of monsoon precipitation was modulated by the El Niño–Southern Oscillation (ENSO), and monsoon–ENSO relationship is related to the variance of ENSO. Spatial correlations between δ18O and May–October precipitation, obtained from the CRU TS4 data set for the period 1901–1999.