We present the spatial variations and isotopic fractionations of the stable isotopic compositions of waters across central Sri Lanka and discuss their applicability as tracers in hydrological studies of the island. The stable isotopic compositions of lake waters in the dry zone of the island were affected by evaporative isotopic enrichment and therefore can be used to estimate the evaporative loss from these lakes. The stable isotopic compositions of stream waters in the wet zone indicate a clear equilibrium isotopic fractionation with altitude (the altitude effect), which is useful in tracing water sources. The isotopic compositions of stream waters in the dry and intermediate zones are higher than expected from their altitude, likely stemming from the outflow of water from upstream tanks or reservoirs that are affected by evaporative isotopic enrichment and are unsuitable for estimating the altitude effect in those areas. The stable isotopic compositions of groundwater and tap and bottled waters plot along the local meteoric water line, suggesting that these waters preserve information on the isotopic compositions of rainwater in their recharge areas. Results indicate that the stable isotopic compositions of surface waters can be an effective tool in the hydrological and hydrogeological studies of the island.