Abstract: Driven by efforts toward carbon-neutral steelmaking, increased scrap usage elevates Sn content in steels. While the general effects of Sn on steel have been studied, its specific influence on resistance spot welding (RSW) remains unclear. This study investigates Sn’s impact on the mechanical properties of RSW joint of 460 MPa HSLA steel. Cross-tension tests reveal that both the RSW joint without Sn and the RSW joint·containing 0.09wt% Sn exhibit pull-out failure. The RSW joint containing 0.09wt% Sn showing higher peak load and energy absorption attributed to Sn’s solid–solution strengthening. Conversely, the RSW joint containing 0.52wt% Sn exhibited the partial interface failure mode, significantly reducing the peak load and energy absorption. The primary reason is the segregation of Sn in the interdendritic regions of the fusion zone, which weakens atomic cohesion and reduces fracture toughness. Such severe segregation arises from RSW’s high cooling rates, which shift the primary solidification phase from δ-ferrite to austenite. Fortunately, double-pulse RSW mitigates Sn segregation, restoring failure mode and mechanical performance. This study assesses the impact of Sn on RSW joint properties, and these findings highlight the broader significance of understanding scrap-related residual element effects in sustainable steel production.