<p>The spot weldability of transformation-induced plasticity (TRIP) steels with varying silicon (Si) and aluminum (Al) contents was evaluated through weld growth curves, microstructural characterization, and mechanical testing. The base metal (BM) exhibited a multiphase structure of ferrite, bainitic ferrite, martensite, and retained austenite (RA), with RA stability increasing as Al content rose. In the heat-affected zone (HAZ), Si-rich steels showed RA decomposition into ferrite and carbides, whereas Al-rich steels retained RA, indicating superior thermal stability. The fusion zone (FZ) consisted mainly of coarse lath martensite, with negligible RA in Si-rich steels and a small fraction of RA in Al-rich steels. Both the cross-tensile strength (CTS) and tensile shear strength (TSS) of the welded joints increased with higher Al and lower Si content in TRIP steels. Failure mode transitioned from interfacial failure (IF) to plug failure (PF) at smaller nugget sizes in Al-rich steels, widening the suitable welding current range. Substituting Si with Al reduced the base metal tensile strength (TS) and the hardness ratio between the weld nugget (HV_n) and base metal (HV_b), which lowered the critical nugget diameter (<i>d</i>_<i>c</i>) required to prevent interface failure. These results demonstrate that Al enhances weldability by stabilizing RA and refining martensite morphology, while Si promotes carbide formation and brittleness in the HAZ.</p>