The Friction Stir Welding procedure (FSW) is a promising approach for a variety of joined aluminum composites. However, using FSW in the welding of aluminum materials is a challenge. In recent years, there has been significant progress. Underwater friction stir welding (UWFSW) is a skilled procedure. It is a completely new and futuristic technology. The non-consumable tool runs over the substrate inside water without emitting any fumes in this method. There are several key advantages, including minimal distortion, a lack of liquefaction caused by imperfections, and excellent joining power. The quality of FSW joints in underwater is determined by factors such as tool rotational speed (TRS), tool transverse speed (TTS), tool tilt angle (TTA), and pin profiles. Each working parameter has a significant impact on joint strength. The purpose of this study is to examine the effect of UWFSW on the mechanical properties and microstructure characteristics of AA5083 armor-grade aluminum. In this work, one of the working characteristics is changed from the lowest to the highest level while the others remain constant. The joint formed from a straight hexagonal tool pin with a TRS of 1500 rpm, TTS of 40 mm/min, and TTA of 1° provided higher tensile strength and hardness than its counterparts due to the induction of stabilized material flow and the configuration of imperfection-free and perfect weld joints. © 2022, ASM International.