This dataset provides monthly amount-weighted precipitation isotope observations from 41 stations across India, collected under the aegis of the National Programme on Isotope Fingerprinting of Waters of India (IWIN), a multi-institutional precipitation isotope monitoring network. It contains 1,034 station-month records from station-specific observation periods falling between June 2005 and September 2016. Each record includes station identifier, station name, WGS84 latitude and longitude, elevation, Köppen-Geiger climate class, monthly rainfall amount, δ18O, δ2H, and d-excess. Rainwater was collected using the IWIN rainwater sampling protocol designed to minimise evaporation. The sampler used a funnel connected to an airtight carboy, with a floating ball to reduce evaporation when rainfall was absent, a wire mesh to exclude coarse debris, a long air-outlet tube to limit air exchange, and a cloth cover to protect collected water from direct sunlight. Two sampling protocols were employed across the network: fortnightly accumulated samples (29 stations), collected on the 1st and 16th of each month, and daily accumulated samples (12 stations), collected each morning following a rainfall event the previous day. Collected water was mixed, measured, transferred to plastic bottles filled nearly to the brim, sealed with inner stubs and pilferage-proof caps, and preserved without chemical additives. Oxygen and hydrogen isotope ratios were measured at the Physical Research Laboratory, Ahmedabad, by continuous-flow IRMS using a Thermo Scientific Delta V Plus with GasBench II. Analytical precision was ±0.1‰ for δ18O and ±1.0‰ for δ2H. d-excess is calculated as δ2H - 8 × δ18O.