Geological investigations of the Precambrian crystalline basement in the Baltic Sea region have been performed on five samples from drill cores or percussion drill fragments collected from 0.2 to 2.5 km depths. The samples were analyzed for geochemistry and dated with the U-Pb zircon methods using SIMS or LA-ICP-MS techniques. A coarse-grained K-feldspar-bearing granitoid at Frigsarve, southern Gotland, has an age of 1845 ± 4 Ma, and can be correlated with the TIB 0 generation of the Transscandinavian Igneous Belt (TIB). The U-Pb age of a quartz monzodiorite at Böda Hamn, northern Öland, was determined to 1799 ± 4 Ma, while a quartz monzonite at Valsnäs, central Öland, yielded 1788 ± 5 Ma; both rocks showing geochemical affinities to the TIB. The Böda Hamn quartz monzodiorite is correlated with the TIB 1a generation while the Valsnäs quartz monzonite is correlated with the TIB 1b generation. The U-Pb ages of granodiorites in the E7-1 and D1-1 drill holes offshore Latvia and Lithuania are 1764 ± 7 Ma and 1744 ± 7 Ma, respectively. Both rocks can be correlated with orthogneisses of the Blekinge Province in southern Sweden as well as orthogneisses in northern Poland. A review of previous correlations of the Precambrian crust in the Baltic Sea region suggests that five crustal units, formed in accretionary environments in the time period from 1.90 to 1.75 Ga, extend from the Fennoscandian Shield in SE Sweden into the Baltic Sea region. The northern part of the Baltic Sea region is dominated by the 1.90–1.87 Ga Svecofennian units continuing into the Precambrian basement beneath Estonia and Latvia. The oldest generation of the Transscandinavian Igneous Belt (TIB 0) can be followed from Askersund in south-central Sweden, via southern Gotland to northwestern Lithuania. The TIB 1 comprises three subunits (TIB 1a–c) which dominate the southwestern parts of the East European Craton from the Svecofennian Domain to the Tornqvist zone.