The data set includes: A) Instantaneous hourly sea level heights measured at the tide gauge on Molo (Pier) Sartorio in the harbour of Trieste, Italy (the station is no. 340 in the GLOSS Core Network). The time series starts on 1 January 1905, 00 hr (UTC+1), and ends on 1 January 2023, 00 hr (UTC+1). B) Monthly mean sea levels from January 1875 to December 2022. C) Annual mean sea levels from 1869 to 2022. Sea-level heights are expressed in centimetres, relative to the tide gauge zero ('Zero Istituto Talassografico', ZIT), which is situated 3.993 m below the internal tide gauge benchmark ('Piastrina mareografica', PM). The most recent quotation of PM dates back to autumn 2002 with 2.3310 m above the IGM (Istituto Geografico Militare) national Italian altimetric datum. The data were digitized from charts until 11 January 2001, and recorded on datalogger afterwards.