Laser beam welding of aluminum alloys often exhibits keyhole-induced porosity and strong weld pool fluctuations that degrade joint integrity. Previous studies have shown that an alternating magnetic field applied from the top of the workpiece can reduce porosity and alter weld geometry. However, direct real-time visualization of the weld pool under alternating fields has not yet been reported, limiting the ability to assess weld pool stability. In this work, the effect of an alternating magnetic field on weld pool behavior during laser beam welding of the aluminum alloy AlMg3 was investigated using in-situ high-speed video imaging. The images were captured coaxially to the welding laser with diode-laser illumination at a separate wavelength. The in-situ imaging was supplemented with ex-situ surface topography measurements and metallographic analysis. The weld pool was consistently shorter and wider under the alternating magnetic field. Temporal fluctuations in the weld pool length, surface height, and penetration depth were reduced. In addition to an up to 91% reduction in the porosity ratio, the standard deviations of the weld pool length, surface height, and the penetration depth were decreased by up to 75%, 41%, and 31%, respectively. The results demonstrate that an alternating magnetic field can expand the stable process window and offer a promising contactless method to improve laser beam weld reliability in aluminum alloys by stabilizing the molten pool and reducing porosity.