Marine carbon dioxide (CO₂) system data has been collected from December 2014 to June 2018 in the Northern Salish Sea (NSS; British Columbia, Canada) and consisted of continuous measurements at two sites as well as spatially- and seasonally distributed discrete seawater samples. The array of CO₂ observing activities included high-resolution CO₂ partial pressure (pCO₂) and pH_T (total scale) measurements made at the Hakai Institute’s Quadra Island Field Station (QIFS) and from an Environment Canada weather buoy, respectively, as well as discrete seawater measurements of pCO₂ and total dissolved inorganic carbon (TCO₂) obtained during a number of field campaigns. A relationship between NSS alkalinity and salinity was developed with the discrete datasets and used with the continuous measurements to highly resolve the marine CO₂ system. Collectively, these datasets provided insights into the seasonality in this historically under-sampled region and detail the area’s tendency for aragonite saturation state (Ω_arag) to be at non-corrosive levels (i.e., Ω_arag > 1) only in the upper water column during spring and summer months. This depth zone and time period of reprieve can be periodically interrupted by strong northwesterly winds that drive short-lived (∼1 week) episodes of high-pCO₂, low-pH, and low-Ω_arag conditions throughout the region. Interannual variability in summertime conditions was evident and linked to reduced northwesterly winds and increased stratification. Anthropogenic CO₂ in NSS surface water was estimated using data from 2017 combined with the global atmospheric CO₂ forcing for the period 1765 to 2100, and projected a mean value of 49 ± 5 μmol kg^-1 for 2018. The estimated trend in anthropogenic CO₂ was further used to assess the evolution of Ω_arag and pH_T levels in NSS surface water, and revealed that wintertime corrosive Ω_arag conditions were likely absent pre-1900. The percent of the year spent above Ω_arag = 1 has dropped from ∼98% in 1900 to ∼60% by 2018. Over the coming decades, winter pH_T and spring and summer Ω_arag are projected to decline to conditions below identified biological thresholds for select vulnerable species.