We present a monthly resolved stable Ba isotope record (D¹³⁸Ba_coral) of a young fossil coral (Porites) from the eastern side of the Andaman Islands (NE Indian Ocean), which grew prior to the 19th century. This D¹³⁸Ba_coral record complements 19 years of monthly resolved Ba/Ca_coral, Sr/Ca_coral, U/Ca_coral, D¹⁸O_coral and D¹³C_coral data from the same colony that can serve as a baseline of environmental variability before the industrialised era. The D¹³⁸Ba_coral record exhibits small but significant seasonal variability ranging from 0.16 to 0.27 ± 0.03‰ over two continuous annual cycles. The D¹³⁸Ba_coral signature is generally low during the South Asian summer monsoon (SAM, June-September) and post-SAM seasons (October-January), which are characterised by high Ba/Ca_coral and more depleted D¹⁸O_SW values. We suggest that Ba desorption from suspended fluvial sediments followed by lateral advection are the main causes of the low D¹³⁸Ba_coral and elevated Ba/Ca_coral values during the SAM and post-SAM. However, this promising Ba proxy behaviour is interrupted by pronounced spikes of low D¹³⁸Ba_coral and high Ba/Ca_coral signatures observed during the pre-SAM season (February-May) throughout the 19-year record. Possible explanations for these spikes observed during dry seasons are (1) Ba enrichment associated with decreased mixed layer depth or (2) Ba release from sediments trapped by local fringing mangroves. Surface seawater from the coral site sampled over an annual cycle exhibits a wide range of dissolved D¹³⁸Ba_SW and [Ba]_SW values, with significantly lowered D¹³⁸Ba_SW of 0.29 ± 0.04‰ and high [Ba]_SW of 66.03 nmol/kg during the SAM, which is broadly consistent with the coral skeletal signals. Our results establish a clear link between monsoon-driven freshening events and Ba isotope variability of surface waters and assess the utility of coral skeletal Ba isotopes to trace riverine inputs into tropical coastal oceans.