The authors have developed an Ir-containing Ni based superalloy tool, strengthened by γ′ precipitation, for friction stir welding (FSW) of steel and titanium alloys. The Ir-containing Ni based superalloy tool exhibited an acceptable performance during FSW of 304 stainless steel and Ti-6Al-4 V. In this study, feasibility of Ir-containing Ni based superalloy tool to friction stir spot welding (FSSW) of high strength steel was examined. FSSW was conducted on the lapped sheets of 1.2 mm-thick DP590 steel. The Ir-containing Ni base superalloy tool with 1.2 mm long probe successfully produced the spot welds on DP590 steel without macroscopic deformation. Even after 600 FSSW trials, the tool shape was hardly changed, suggesting that the Ir-containing Ni based superalloy tool is appropriate for FSSW of high strength steel.