The data set includes daily near-surface sea temperatures measured in the harbour of Trieste (Italy) during 1899-2015. Two time series are formed: the first is derived from analogue measurements performed in the 1899-2008 period, the second is derived from digital records in 1986-2015. From these time series a composite time series for 1899-2015 is built. Data records consist of date and daily temperature in degrees Celsius to the 0.1 °C precision.