Groundwater quality in Sri Lanka shows wide variations due to local and regional-scale geo-climatic changes. Here, we present groundwater hydrogeochemical and dissolved CO₂ variations in newly defined hydro-geo-climatic regions in Sri Lanka. Summary parameters with electrical conductivity (EC) and total hardness (TH) were used as primary indicators for 1318 groundwater samples from shallow aquifers spread over the country. Furthermore, we calculated dissolved CO₂ in groundwater (n = 667) with temperature, pH and alkalinity. Our analysis indicates that groundwater in the dry zone showed higher median EC and TH (740 µScm^-1 and 199 mg L^-1) values than in the intermediate (400 µScm^-1 and 120 mg L^-1) and wet zones (146 µScm^-1 and 53 mg L^-1). Groundwater in the northern sedimentary terrain showed extreme EC of up to 34000 µScm^-1 and TH of up to 3819 mg L^-1 with medians of 1006 µScm^-1 and 261 mg L^-1, respectively. Because of the low weathering rate of the basement minerals in the central part of Sri Lanka (Highland complex), groundwater showed the lowest EC and TH values compared to the other regions. Groundwater CO₂ values were varied from 0.01 to 4.82 mmol L^-1 and the wet zone-Wanni complex showed a higher median (1.06 mmol L^-1), while the dry zone-Vijayan complex showed a lower median (0.08 mmol L^-1). Future increased monitoring would also benefit groundwater management issues in relation to climate change and population growth in the country.