Streptococcus pyogenes is a species of Gram-positive, aerotolerant bacteria in the genus Streptococcus. These bacteria are extracellular, and made up of non-motile and non-sporing cocci (round cells) that tend to link in chains. They are clinically important for humans, as they are an infrequent, but usually pathogenic, part of the skin microbiota that can cause group A streptococcal infection. S. pyogenes is the predominant species harboring the Lancefield group A antigen, and is often called group A Streptococcus (GAS). However, both Streptococcus dysgalactiae and the Streptococcus anginosus group can possess group A antigen as well. Group A streptococci, when grown on blood agar, typically produce small (2–3 mm) zones of beta-hemolysis, a complete destruction of red blood cells. The name group A (beta-hemolytic) Streptococcus is thus also used.

The species name is derived from Greek words meaning 'a chain' (streptos) of berries (coccus [Latinized from kokkos]) and pus (pyo)-forming (genes), since a number of infections caused by the bacterium produce pus. The main criterion for differentiation between Staphylococcus spp. and Streptococcus spp. is the catalase test. Staphylococci are catalase positive whereas streptococci are catalase-negative. S. pyogenes can be cultured on fresh blood agar plates. The PYR test allows for the differentiation of Streptococcus pyogenes from other morphologically similar beta-hemolytic streptococci (including S. dysgalactiae subsp. esquismilis) as S. pyogenes will produce a positive test result.

An estimated 700 million GAS infections occur worldwide each year. While the overall mortality rate for these infections is less than 0.1%, over 650,000 of the cases are severe and invasive, and these cases have a mortality rate of 25%. Early recognition and treatment are critical; diagnostic failure can result in sepsis and death. S. pyogenes is clinically and historically significant as the cause of scarlet fever, which results from exposure to the species' exotoxin.

Unlike most bacterial pathogens, S. pyogenes only infects humans. Thus, zoonotic transmission from an animal (or animal products) to a human is rare.

S. pyogenes typically colonizes the throat, genital mucosa, rectum, and skin. Of healthy adults, 1% to 5% have throat, vaginal, or rectal carriage, with children being more common carriers. Most frequently, transmission from one person to another occurs due to inhalation of respiratory droplets, produced by sneezing and coughing from an infected person. Skin contact, contact with objects harboring the bacterium, and consumption of contaminated food are possible but uncommon modes of transmission. Streptococcal pharyngitis occurs most frequently in late winter to early spring in most countries as indoor spaces are used more often and thus more crowded. Disease cases are the lowest during autumn.

Maternal S. pyogenes infection usually happens in late pregnancy, at more than 30 weeks of gestation to four weeks postpartum. Maternal infections account for 2 to 4% of all clinically diagnosed S. pyogenes infections. The risk of sepsis is relatively high compared to other bacterial infections acquired during pregnancy, and S. pyogenes is a leading cause of septic shock and death in pregnant and postpartum women.

In 1928, Rebecca Lancefield published a method for serotyping S. pyogenes based on its cell-wall polysaccharide, a virulence factor displayed on its surface. Later, in 1946, Lancefield described the serologic classification of S. pyogenes isolates based on components of their surface pili (known as the T-antigen) which are used by bacteria to attach to host cells. As of 2016, a total of 120 M proteins have been identified. These M proteins are encoded by 234 type emm genes with greater than 1,200 alleles.

All strains of S. pyogenes are polylysogenized, in that they carry one or more bacteriophage in their genomes. Some of the phages may be defective, but in some cases active phage may compensate for defects in others. In general, the genome of S. pyogenes strains isolated during disease are >90% identical, they differ by the phage they carry.