<p>A356 (Al–Si–Mg) alloy is widely used in automotive industry; however, it suffers limited ductility in as-cast conditions. This drawback has traditionally been overcome using chemical modification by Na or Sr additions, heat treatment, or solid-state friction stir processing (FSP). In this study, the synergizing effects of strontium modification during casting and in-process water cooling during FSP were systematically investigated on the final microstructure and mechanical properties of cast A356 alloy. For this purpose, plates of standard and Sr-modified A356 alloys were prepared by melting in an induction furnace in air followed by gravity casting into a Y-block steel mold. The cast plates were subsequently subjected to FSP in air and underwater cooling conditions, while all other FSP parameters were kept constant. Microstructure and mechanical properties of the FSPed samples were studied and compared with those of the cast base metals. The findings indicate that Sr addition modifies the Si needles morphology to fine fibrous morphology which enhances both strength and ductility. FSP further refines the microstructure, reduces the hardness and slightly strength but significantly increases the ductility of SZ compared to the base metals. The sample from the Sr-modified base metal which FSPed under water cooling showed the most refined microstructure and superior mechanical properties among the FSPed samples. In this sample, the average Si particle size, tensile strength, and fracture strain were 1.2&#xa0;mm, 170&#xa0;MPa, and 28.4%, respectively; corresponding to a 37% reduction in particle size, an 8% decrease in strength, and a 209% increase in ductility compared with the corresponding base metal. The study demonstrates how combining Sr addition with in-process water cooling during FSP creates synergistic improvements in aluminum casting performance.</p>