Abstract Marine heatwaves (MHWs) are increasing in frequency and intensity in global boundary current systems such as the Gulf Stream (GS), where ocean circulation regulates upper‐ocean heat and water‐mass properties. We examine temperature–salinity co‐evolution during MHWs in the Northwest (NW) Atlantic using satellite observations and ocean reanalysis for 2012–2023. Applying a consistent threshold‐based framework to sea surface temperature and sea surface salinity (SSS), we identify co‐occurring MHWs and salinity extremes and evaluate their spatial extent and vertical structure. Results reveal a statistically significant high‐salinity imprint during MHWs along the GS North wall and shelf‐break corridor, consistent with advection of warm, saline GS waters preconditioning the upper ocean for vertically extensive warming. Low‐SSS extremes tend to cover greater area on the continental shelf, where surface freshening likely favors shallow, surface‐intensified MHWs. These results underscore SSS as a critical diagnostic for advective water‐mass influence and stratification during MHWs in the NW Atlantic.