We present a statistically robust reconstruction of Thailand's Chao Phraya River peak season streamflow (CPRPF) that spans the 202 years from 1804 to 2005 CE. Our reconstruction is based on tree ring δ18O series derived from three Pinus merkusii sites from Laos and Thailand. The regional δ18O index accounts for 57% of the observed variance of CPRPF. Spatial correlation and 21‐year running correlation analyses reveal that CPRPF is greatly influenced by regional precipitation variations associated with the El Niño–Southern Oscillation (ENSO). Periods of enhanced and reduced ENSO activity are associated with strong and weak ENSO‐streamflow correlation, respectively. At the longer timescale, the Pacific Decadal Oscillation (PDO) appears to modulate the ENSO‐streamflow correlations, with the most extreme flood events along the Chao Phraya River occurring during periods of increased frequency of La Niña events that coincide with extended cold phases of the PDO. The CPRPF reconstruction could aid management planning for Thailand's water resources.