Streptococcus agalactiae 1a ST7 CC1 is an important emerging pathogen in tilapia aquaculture in Latin America. This bacterium, which belongs to the group B streptococcus (GBS), has been associated with severe outbreaks in tilapia farms in Central and South America, resulting in high mortality and economic losses. The objective of this study was to genetically and phenotypically characterize 36 isolates of S. agalactiae 1a from field outbreaks in six Latin American countries. Using MLST, the 36 isolates were identified as S. agalactiae serotype 1a sequence type 7 (ST7), which belongs to clonal complex 1 (CC1), characterized by high virulence and adaptability to aquatic environments. The results of antibiograms and Minimum Inhibitory Concentration (MIC) show variable results depending on the geographical region, but in general there is a group sensitive to florfenicol and amoxicillin, but another group resistant to florfenicol and sensitive to amoxicillin. Despite these phenotypic results, the expression of antimicrobial resistance genes was only evident for oxytetracycline (tetM, tetO), but not for flenicols (fexA, fexB), erythromocin (ermB, ermTR) or lincomycin and clindamycin (linB). Expression of virulence genes (spb1, scpB, bca, bac, dtlR, cfb, sodA) by RT-PCR showed a variable pattern between geographical areas, but none were classified in the profiles described in the literature. Understanding the pathogenic mechanisms, pathogen-host interactions, immune response, genotypic and phenotypic characterization of field isolates, and environmental factors influencing S. agalactiae 1a ST7 CC1 infections is crucial for developing effective prevention and control strategies in tilapia aquaculture, highlighting the need for further research and improved biosecurity measures to mitigate the impact of this pathogen.