Thin-walled aluminum sections are widely used as energy absorbers because of their low weight and ability to dissipate energy through plastic deformation. This study experimentally and numerically investigated the effects of cross-sectional shape and the number of longitudinal weld lines on the axial crushing behavior of thin-walled aluminum sections. Square specimens measuring 35 × 35 mm and rectangular specimens measuring 55 × 35 mm, all with a wall thickness of 2 mm and a height of 100 mm, were tested in unwelded, two-line welded, and four-line welded configurations. Quasi-static axial compression tests were performed at a loading rate of 400 mm/min. In the numerical analysis, the welding-induced temperature field and residual stresses were simulated in ABAQUS and then transferred to the crushing model. The results showed that longitudinal weld lines changed the initiation and propagation of folds and reduced the initial peak force. The numerical initial peak forces of the unwelded square and rectangular specimens were 45 and 43 kN, respectively. These values decreased to 37 and 40 kN for the two-line welded specimens and to 36 and 38 kN for the four-line welded specimens. The four-line welded rectangular specimen also exhibited the highest residual-stress level. The agreement between the experimental and numerical force–displacement responses and crushing modes confirmed the capability of the model to predict the overall behavior of the specimens. Overall, cross-sectional geometry and weld-line number influenced the initial peak force and axial crushing response.