High-frequency measurements of surface water partial pressure of CO2 (pCO2) in the Gulf of Trieste in the north Adriatic Sea, the most northern part of the Mediterranean, are presented. Sea surface pCO2 was measured at the coastal oceanographic buoy Vida (45° 32′ 55.68′′ N, 13° 33′ 1.89′′ E), using the Submersible Autonomous Moored Instrument for CO2 during four deployments from 2007 to 2008. The pCO2 measurements were combined with physical observations at Vida, Sea-viewing Wide Field-of-view Sensor chlorophyll-a, and daily river discharge to elucidate the biological and physical processes that control pCO2 on seasonal to interannual time scales. Sea surface pCO2 ranged from 260 to 340 μatm in spring 2007, 220 to 380 μatm in late summer/fall 2007, 200 to 500 μatm in spring/summer 2008, and 240 to 370 μatm in fall 2008. Mean CO2 fluxes (±standard deviation) from the atmosphere into the ocean during these seasons were −5.0 (±6.2), −12.1 (±14.2), −0.9 (±9.7), and −6.4 (±7.0) mmol m−2 d−1 resulting in a net sink of atmospheric CO2 of −2–3 mol m−2 yr−1. The seasonal and interannual variability in this semienclosed sea is dominated by heating and cooling based on strong correlations with sea surface temperature. These correlations break down during convective mixing in the fall, bora wind events, and brief periods of freshwater input.