Necrotizing soft tissue infections (NTSIs) represent a concept of necrotizing infections involving the skin, subcutaneous tissue, fascia, and muscle, and it is a potentially fatal disease. Early exploratory incision is strongly recommended for both the diagnosis and treatment of necrotizing soft tissue infections. Treatment of necrotizing soft tissue infections requires the administration of appropriate antimicrobial agents and adequate surgical debridement. The emergence of M1(UK)-lineage Streptococcus pyogenes (S. pyogenes) is recently reported in the UK, Canada, the USA, and the Netherlands. We report a Japanese case of sartorius muscle (SM) NTSI caused by M1(UK)-lineage S. pyogenes. A 34-year-old man developed redness and swelling of his right thigh anterior compartment with fever in October 2024. The closed and deep effusions by active exploratory incision/debridement on hospital days one to three yielded the presence of Gram-positive cocci, although two sets of blood cultures upon admission revealed no bacterial growth; its species identification results indicated S. pyogenes. Clinical and pathological diagnosis was streptococcal SM NTSI (without toxic shock syndrome (TSS)). Negative pressure wound therapy with instillation and dwelling (NPWTi-d) to promote his soft tissue cure was performed along with antimicrobial regimens. The patient recovered and received micrografting (carrier: artificial dermis made from collagen sponge and silicon film, Pelnac Gplus® (Gunze Limited, Osaka, Japan) using the Rigenera® system (Rignera HBW, Candiolo, Italy). Thereafter, he developed bronchitis: the sputum yielded S. pyogenes growth: he recovered uneventfully. Split-thickness skin grafting (STG) was performed. Split-thickness skin grafting was fully engrafted, and the wounds achieved complete healing. The patient could walk by himself. Microbiological genetic analyses using both DNAs from effusion/sputum-origin strains revealed the emm1.0 and speA-speB-smeZ profiles. rofA-gldA-pstB sequencing results indicated M1(UK)-specific single-nucleotide polymorphisms. The streptococcal inhibitor of the complement-mediated lysis gene allele was the streptococcal inhibitor of the complement-mediated lysis-1.02 allele. Micrografting using the Rigenera® system and STG following NPWTi-d can be beneficial approaches. Clinicians should perform cultures using sterile specimens (deep effusions/tissues) from infection foci through exploratory incision/debridement, along with two sets of blood cultures, when examining patients with/without underlying medical conditions.