A feasibility of friction stir welding for a high‐strength low‐alloy steel subjected to tempforming, that is, tempering followed by large‐strain warm rolling at tempering temperature, is presented. The martensite microstructure tempered at 873 K for 1 h and then subjected to warm rolling to total strain of 1.5 is characterized by a lamellar‐type ultrafine‐grained microstructure consisting of pan‐caked grains with the transverse size of 560 nm and exhibits the tensile strength of 1020 MPa. The friction stir welding is accompanied by heating the stir zone of the welded sample well above Ac1 temperature. Hence, the welding seam experiences austenite reversion followed by martensitic transformation. The microstructure evolved in the stir zone is composed of near‐equiaxed grains with an average size of 1.3 μm. Nevertheless, the stir zone possesses increased hardness up to 350 HV as compared to that of 290 HV in the base material after tempforming. Upon tensile test of the welded joint, the strain localization followed by fracture occurs in the heat‐affected zone close to the stir zone. The tensile strength of 970 MPa is obtained for the welded joint that is quite close to the base material after temforming.