This dataset includes maximum contents of 137Caesium in subsurface sediments determined for 1-cm sediment slices of 55 short gravity-cores from the Gulf of Bothnia, northern Baltic Sea. Sediment cores recovered consist primarily of soft, organic-rich, silty clay or clayey silt. Radioactivity from 137Cs in sediments has generally declined due to radioactive decay of 137Cs over the last decades. However, 137Cs contents in subsurface sediments remain at elevated levels compared to pre-Chernobyl levels (i.e. levels before the 1986 Chernobyl nuclear power plant accident). The 137Cs activity of untreated samples was measured by gamma spectrometry using an EG&E Ortec ACE™-2K spectrometer with a four-inch NaI/TI detector at the Geological Survey of Finland (GTK), Espoo. Since 2017, the 137Cs activity of sediment samples has been measured for 60 min using a BrightSpec bMCA-USB pulse height analyser coupled to a well-type NaI(Tl) detector at the Geological Survey of Finland. The 137Cs activity contents are presented per wet weight (Bq kg-1). Samples collected and measured over different years from 2001 to 2019 gave 137Cs values representing discrete time frames during the 18-year sampling/measuring interval, and are not fully, directly comparable. Thus, dataset lists also 137Cs activity content values decay corrected to 2021.