The past climate in northwestern Thailand remains insufficiently understood because of the limitation of climate proxies. We present a new record of paleoclimate activity during 2050–1551 years BP (before the present), based on the analysis of the oxygen isotope ratios (δ¹⁸O) of tree-ring cellulose in ancient teak log coffins excavated from Namjang Cave in Mae Hong Son Province, northwestern Thailand. The ages of the teak log coffin samples were determined using C-14 dating. The average CoffinNJ δ¹⁸O value was 23.48 ± 0.77‰, values ranging from 21.23 to 25.42‰. The mean May–October (MO) rainfall reconstructed from the CoffinNJ δ¹⁸O data was 274 mm. The mean May–October (MO) rainfall data showed significant negative correlations with stalagmite δ¹⁸O data from Laos (r = − 0.254, p < 0.01), highlighting the existence of a weak monsoon from 291 to 294 AD and strong monsoon in 132 AD and 142 AD. Additionally, the MO rainfall data showed a significant positive correlation with reconstructed rainfall in Tibet (r = 0.347, p < 0.01). Spectral analysis of the CoffinNJ δ¹⁸O values revealed centennial cycles related to the sunspot number. The CoffinNJ δ¹⁸O values have a positive significant correlation with the sunspot number (r = 0.410, p < 0.01) over the entire period. Moreover, we found a highly significant positive correlation (r = 0.644, p < 0.01) between the CoffinNJ δ¹⁸O values and stalagmite δ¹⁸O values from Wanxiang Cave, China, and this correlation is related variations in the Asian monsoon. We conclude that the CoffinNJ δ¹⁸O data reflect the Asian monsoon from 2000 years ago and have the potential to be a paleoclimate proxy in northwestern Thailand.