Surface pCO₂ is of great interest for the calculation of air-sea CO₂ fluxes, uptake of anthropogenic CO₂, and eventually the Global Carbon Budget. However, underlying data are usually scarce in space and time and require inter- and/or extrapolation. Compared to the open ocean, variability in coastal ecosystems can be particularly large, related to intense productivity and high turnover rates. As a result, the impact of coastal seas on the CO₂ budget and their source or sink status is often associated with high uncertainties or even remains unclear. Here, we present a new monthly climatology of surface pCO₂ for the coastal Baltic Sea. On one side, we use accurate surface pCO₂ data from ship-of-opportunity (SOOP) observations between 2003 and 2021. On the other side, we developed a novel extrapolation approach that exploits ecosystem model data of pCO₂ variability to inform the mapping. It provides both an estimate of mapped pCO₂ as well as of mapping uncertainty for the climatology. At the same time, the uncertainty data can help to identify gaps in observation networks and to improve monitoring strategies. Over the past 2 decades, we observe increasing surface pCO₂ (ca. +1.5 µatm yr^–1) in the shallower Southern Baltic Sea, while no significant temporal trend is discernible in the Central Baltic Sea. Compared to higher trend estimates for previous periods in the literature, this may be indicative of a reduction of CO₂ source strength or eventually transition into (an increasing) CO₂ sink of this coastal sea.