High salinity Ca–Na–Cl brines occur in Sweden and Finland. The complex geological history of the Fennoscandian Shield is reflected by the wide range of saline waters whose chemical compositions have been explained by local water–rock interaction, simple evolution of relict sea water, or freezing of sea water during glacial periods. This study deals with the present knowledge of Sr isotopes in groundwater from the Fennoscandian Shield to better constrain the origin of their deep saline component. Two different mixing trends are seen between Äspö Island groundwater and the Baltic Sea. The first trend links the North Sea to rivers through various dilution levels in the Baltic; the second trend is a simple linear relationship through Äspö groundwaters, agreeing with a binary mixing between a Baltic end-member and a highly Sr-concentrated fluid. Strontium isotope compositions are presented for precipitation in Finland and Sweden, the Baltic Sea, and the river waters draining the Fennoscandian Shield, and for groundwaters from several sites in the Fennoscandian Shield. At least six mixing lines between rain-and-snow input and surface waters can be drawn in each region around the Baltic Sea, reflecting a first step of water–rock interaction. A second series of calculated lines links the surface end-members and the deep brines, and defines a range of 87Sr/86Sr ratios for the deep brines at each site. All sites show a specific 87Sr/86Sr signature and the large 87Sr/86Sr variations is site specific. The well-constrained 87Sr/86Sr signatures in deep brines should correspond to a large, well-mixed, and homogeneous water reservoir.