Abstract: To mitigate fatigue susceptibility in laser-welded high-strength wheel steel joints for automotive lightweighting, this study investigates the effect of 0.016 wt.% cerium (Ce) on microstructural evolution and mechanical properties. Ce addition increases the average fatigue life by ~2.1-fold at 360 MPa, improves impact toughness in all joint regions, and narrows the heat-affected zone (HAZ) width from 2.28 to 1.16 mm. It also promotes a microstructural transition from upper bainite (UB) and ferrite side-plates (FSP) to a tougher configuration dominated by acicular ferrite (AF) and lower bainite (LB). The underlying mechanism is associated with the dual occurrence modes of Ce, partitioned into ~0.0050 wt.% in solid solution and ~0.0104 wt.% in compound form. In-situ HT-CLSM shows that Ce suppresses prior-austenite grain-boundary migration, reducing the migration velocity from ~77 to ~10 nm·s⁻¹. Cs-corrected HR-TEM indicates Ce-enriched nanoscale cluster-like regions near prior-austenite grain boundaries, consistent with a solute-drag effect, while finely dispersed Ce2O2S particles impose Zener pinning and provide heterogeneous nucleation sites for AF. The synergistic effects of solute Ce and compound-state Ce refine the HAZ microstructure, improve toughness, and shift fatigue crack initiation from inclusions to the weld toe, thereby enhancing the performance of laser-welded joints.