The northern Gulf of Mexico (nGOM) is a river‑dominated marginal sea with strong physical‑biogeochemical variability. We reconstruct sea surface partial pressure of CO₂ (<em>p</em>CO₂) at 4‑km, 8-day resolution from 2003 to 2024 using a satellite‑based, season‑specific random forest model (independent validation R&sup2; = 0.82, RMSE = 27.6 &mu;atm). The climatological <em>p</em>CO₂ distribution exhibits a sharp coastal‑to‑offshore gradient: river‑influenced coastal waters (SSS &lt; 33) have persistently low <em>p</em>CO₂ with high spatial variability, while offshore waters (SSS &gt; 33) have higher <em>p</em>CO₂ with weaker heterogeneity and lower seasonal amplitude. The nGOM acts as a net CO₂ sink for atmospheric, largely concentrated in the river‑influenced plume region due to riverine nutrient‑stimulated biological uptake.&nbsp;Seasonal <em>p</em>CO₂ variation is dominantly controlled by temperature but counteracted by spring‑summer biological drawdown (reducing <em>p</em>CO₂) and autumn‑winter vertical mixing with CO₂‑rich deeper water (raising <em>p</em>CO₂). Interannual <em>p</em>CO₂ variability is dominantly affected by year-to-year changes in river discharge and nutrient loading, with higher discharge leading to lower <em>p</em>CO₂ via enhanced biological uptake. On a decadal timescale, sea surface <em>p</em>CO₂ increased at a rate of 0.50 &plusmn; 0.20 &mu;atm yr^-1, much slower than atmospheric <em>p</em>CO₂ (2.13 &plusmn; 0.04 &mu;atm yr^-1), leading to a strengthening<strong> </strong>oceanic CO₂ sink with the sea-to-air flux becoming more negative at &minus;0.41 &plusmn; 0.06 mmol C m^-2 d^-1 yr^-1. Furthermore, a decreasing frequency of easterly winds has reduced the westward transport of the Mississippi River plume, causing a higher <em>p</em>CO₂ increasing rate on the western Texas‑Louisiana shelf.