Relevance. South-Eastern Transbaikalia is known by numerous saline lakes with rare earth elements concentrations up to 1 mg/dm³, close to industrially significant levels. Groundwater is the main water source for these lakes. This study investigates the geochemistry of rare earth elements in the regional groundwater, supported by a database of 60 samples, crucial for evaluating the area ore-generating potential. Aim. To analyze rare earth elements geochemistry in the groundwater of South-Eastern Transbaikalia, focusing on sources, migration, fractionation, and the mechanisms of water formation. Objects. Groundwater samples from the upper dynamic zone (60 total), comprising 1 carbonated spring, 3 atmospheric precipitation samples, 10 saline lake samples, and one from the Uldza River. Methods. Rare earth elements concentrations were measured using ICP-MS, and species concentrations were calculated using GWB application, based on equilibrium constants. Results and Conclusions. Rare earth elements concentrations in the groundwater of South-Eastern Transbaikalia is primarily determined by the rare earth elements chemical properties in a specific hydrochemical environment and the water–rock interaction as water exchange slows down. The rare earth elements accumulation in water occurs in one direction with the formation of Na-HCO₃ groundwaters in the region, and once saturation with carbonates is reached, the rare earth elements concentrations may decline together with equilibrium minerals. The rare earth elements migration in groundwater predominantly occurs in the Ln(CO₃)₂^– species, while in in the carbonated spring the Ln(CO₃)⁺ form is predominated, with the proportion increasing along the La-Lu series depending on pH, Eh, TDS, etc. of groundwater. Additionally, in carbonated waters, significant complexes with simple ionic species are formed by light rare earth elements, their concentrations decreasing along the La-Lu series. The rare earth elements distribution largely reflects the pattern in the magmatic rocks of the primary feeding area. It is suggested that the possible rare earth elements sources input into the waters are aluminosilicates, particularly feldspars, specifically celsian, as well as monazite found in Eastern Transbaikalia.