The solid phase joining technique of friction stir welding (FSW) has been shown to mitigate helium-related issues in fusion welding of irradiated steels. Here, we present detailed characterization results of the first friction stir weld of an irradiated 304L stainless steel (SS) coupon contained 5.2 atomic parts per million (appm) helium. Scanning electron microscopy (SEM) was used to characterize a cross-section specimen extracted from the friction stir weld. No macro helium-induced cracks were observed at the analyzed cross-section, and only limited number of 1 – 1.5 μm-sized pores were found in the stir zone (SZ) and the thermal-mechanical affected zone (TMAZ). Few minor helium aggregations (Maximum ~20 μm in length) appeared along grain boundaries inside the TMAZ. Thereafter, miniature tensile specimens were extracted from the SZ, TMAZ and heat affected zone (HAZ), and base metal (BM) and tested. All welded specimens showed high total elongations (> 60%) and strengths (> 75% BM strengths). Overall, the weld made by FSW showed much better results comparing with traditional fusion welding on irradiated SS.