Diabetes mellitus is a major challenge for global health, and Bougainvillea spectabilis Willd. (B. spectabilis) is a widely used herbal remedy with diverse cultivars traditionally used for diabetes treatment. However, the comparative efficacy of these cultivars remains ambiguous. This study aimed to evaluate the D-pinitol content and DPPH radical-scavenging activity of methanolic leaves extracts of five B. spectabilis cultivars. Furthermore, the effects of these cultivars on various parameters, including blood glucose levels, oxidative stress markers, inflammatory cytokines, lipid profiles, liver enzymes, renal function markers, and histopathological changes, were assessed in STZ-induced diabetic rats after one month of oral daily treatment. All tested cultivars demonstrated significant improvements in the measured parameters, albeit to varying extents. Notably, the LOE cultivar, distinguished by its orange bracts, exhibited the highest efficacy, surpassing the effectiveness of glibenclamide, an antidiabetic medication, and displayed the highest concentration of D-pinitol. These findings underscore the importance of carefully selecting the appropriate B. spectabilis cultivar to maximize the antidiabetic efficacy, with a particular emphasis on the correlation between antidiabetic activity and D-pinitol concentrations.