In the present work, sheets of high-density polyethylene, reinforced with strips of polypropylene using a friction stir welding technique were executed. Welding was carried out using a friction stir welding tool of 20 mm shoulder diameter and 5 mm for both pin diameter and pin length with zero tilt angle, the percentages of polypropylene added to the welding zone were 15, 20, 25, 30% (as a percentage of the added polypropylene to the welding zone), the recommended high tool rotation speed and low tool travel speed (520 rpm, 20 mm/min, respectively) were applied in all tests, the plunge depth was 0.5 mm (the penetration depth of tool shoulder from workpiece surface), dwell time at the event of submerging the pin into the faying surfaces and before initiating the tool travel speed was 45 seconds. Mechanical tests, represented by flexural and impact tests, exhibited an improvement in the mechanical properties of the welded specimens for the case of 25% added polypropylene. Friction stir welding has extraordinary potential to create imperfection-free joints and to initiate a high-quality weldment of high-density polyethylene sheets reinforced by polypropylene strips.