Continental shelf seas play an important role in the global carbon cycle. At the interface between the land and deep open ocean, shelf seas are typically highly biologically productive, major sites of organic carbon burial in sediments, and are also highly impacted by human activities. To investigate the marine carbon cycle and its variability through space and time, consistent and quality-controlled data products of marine carbonate system variables, such as the Global Ocean Data Analysis Project (GLODAP), are needed. However, GLODAP focuses on the open ocean, limiting its utility for understanding the marine carbonate system in shelf seas. The northwest European shelf (NWES) seas are of particular interest because of their biogeochemical heterogeneity and expected capacity to absorb, export and bury carbon. Here, we present a new data product for this region called the Northwest European Shelf Data Analysis Product (NWESDAP). NWESDAP includes discrete measurements of marine carbonate system variables (total alkalinity, dissolved inorganic carbon and pH) as well as physical and other biogeochemical variables between 43.6° N and 70° N on the NWES and down to 1,000 m depth. NWESDAP is a compilation of datasets of discrete observations (i.e., laboratory measurements of seawater samples, rather than autonomous sensor measurements) from research cruises and time-series stations from 1975 to 2026. Data from rivers, estuaries and fjords are also included. NWESDAP consists of 46,545 data points (of which 5,221 were in GLODAPv2.2023) from around 500 different expeditions and platforms, merged into a consistent format. Data quality flags corresponding to quantified systematic and random uncertainties have been assigned to each dataset depending on the measurement method used and other available metadata provided. Since GLODAP-style quality control (QC) based on deep ocean cross-over analysis is not possible in the shallow NWES seas (mostly < 200 m), a secondary QC based on variable distributions and relationships has been developed to identify suspected erroneous data points and check for systematic biases in specific datasets, aiming to assess the internal consistency of the entire product with respect to the Global Ocean Acidification Observing Network (GOA-ON) climate and weather quality goals so it can be used to investigate the highly variable marine carbon cycle of the NWES. Of the 46,545 datapoints, 24,822 are of at least weather quality, but the rest has been retained as they can still provide important historical context. The data product and associated metadata are available online at <a href="https://doi.org/10.5281/zenodo.23555137" target="_blank" rel="noopener">https://doi.org/10.5281/zenodo.23555137</a> (Brandon et al., 2026b). We expect this data compilation to facilitate studies of marine carbonate system variability through space and time across the NWES and support the assessment of carbon stocks and fluxes. NWESDAP will also provide a foundation for evaluating the impact of climate change and anthropogenic activities on the carbonate system as well as for validating regional biogeochemical models.