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ABSTRACT Streptococcus pneumoniae (the pneumococcus) is a human pathogen responsible for a spectrum of diseases such as pneumonia, sepsis, and meningitis. The capsule is the major pneumococcal virulence factor and is encoded by the capsular polysaccharide (cps) locus, a recombination hotspot that has resulted in over 100 distinct capsular polysaccharide types (serotypes) identified to date. Recently, 33X (also known as 10X) was proposed as a putative novel serotype, but the capsule structure had not been elucidated. Here, we provide an in-depth investigation of 33X, demonstrating it is a new pneumococcal capsular serotype. In this study, we screened 12,850 nasopharyngeal swabs from both healthy children and pneumonia patients (adults and children) in Mongolia collected between 2015 and 2022. We identified 20 pneumococcal 33X isolates. Using whole genome sequencing, we found that the 33X cps locus is a chimera of genes from pneumococcal serogroups 35, 10, and 33, as well as other Streptococcal species. Serotyping of 33X pneumococci by the Quellung reaction revealed a unique serological profile, typing as both 10B and 33B. Competitive ELISAs confirmed that antibodies that were generated in mice directed against 33X were inhibited by 33X pneumococci but not 10B or 33B. Lastly, the elucidation of the 33X capsule structure revealed that the polysaccharide is distinct from other serotypes, consisting of an O-acetylated hexasaccharide repeat unit of →5)-β-Galf-(1→3)-β-Glcp-(1→5)-β-Galf 2Ac-(1→3)-β-GalpNAc-(1→3)-α-Galp-(1→4)-Rib-ol-(5→P→. Therefore, 33X meets the requisite genetic, serological, and biochemical criteria to be designated as a new serotype, which we have named 33G. IMPORTANCE Streptococcus pneumoniae (the pneumococcus) is a bacterial pathogen with the greatest burden of disease in Asia and Africa. The pneumococcal capsular polysaccharide has biological relevance as a major virulence factor as well as public health importance as it is the target for currently licensed vaccines. These vaccines have limited valency, covering up to 23 of the >100 known capsular types (serotypes) with higher valency vaccines in development. Here, we have characterized a new pneumococcal serotype, which we have named 33G. We detected serotype 33G in nasopharyngeal swabs (n = 20) from children and adults hospitalized with pneumonia, as well as healthy children in Mongolia. We show that the genetic, serological, and biochemical properties of 33G differ from existing serotypes, satisfying the criteria to be designated as a new serotype. Future studies should focus on the geographical distribution of 33G and any changes in prevalence following vaccine introduction.