Six serotypes (Ia, Ib, II, III, IV, and IV) cause nearly all group B streptococcal (GBS) disease globally. Several studies have shown that anti-capsular polysaccharide IgG concentration is associated with risk of invasive GBS disease, however the lack of a standardized assay to measure anti-CPS IgG has precluded comparison of protective antibody concentrations across these studies, or to immunogenicity data from capsular polysaccharide conjugate vaccine clinical trials. A multiplex Luminex-based direct immunoassay (dLIA) has been developed to simultaneously measure the concentration of serum IgG specific for the six prevalent GBS CPS serotypes. The assay uses poly-L-lysine (PLL)-conjugated CPS that is chemically coupled to spectrally distinct Luminex microspheres. Assay validation was performed using serum samples obtained from human subjects vaccinated with an investigational 6-valent GBS CPS conjugate vaccine. Assay results are expressed as IgG concentrations (µg/mL) using a human serum reference standard composed of pooled sera from vaccinated subjects. Weight-based IgG assignments were established using surface plasmon resonance (SPR) to generate a homologous calibrator for calibration in the dLIA. The lower limits of quantitation (LLOQ) for all serotypes covered in the 6-plex GBS IgG dLIA fell within the range of 0.002-0.022 µg/mL IgG. Taken together, the 6-plex GBS IgG dLIA platform is specific for six GBS serotypes included in Pfizer’s investigational vaccine, has a wide dilution adjusted assay range and is precise (<18.5% relative standard deviation) for all serotypes. The 6-plex GBS IgG dLIA permits direct cross-serotype comparison of CPS-specific IgG, and in combination with the validation presented in this work, positions the assay as a powerful tool to serve as the primary serological readout for global seroepidemiology studies and vaccine clinical trials.