Weight-control dietary supplement use among female vocational students may be influenced by sociocultural pressures related to body image and may pose potential health risks when products are used inappropriately. Health literacy may strengthen their ability to access, understand, evaluate, and apply health information when making decisions about dietary supplement use. This study evaluated the effectiveness of a health literacy-based intervention (3 × 3 × 3 Program) in promoting health literacy and safer weight-control dietary supplement use among female vocational students in Bangkok, Thailand. A quasi-experimental study was conducted among 76 female vocational students aged 18–21 years assigned to an intervention group (n = 38) or a control group (n = 38). The intervention group participated in the 3 × 3 × 3 Program, comprising three structured learning sessions, each incorporating three learning steps over three hours, based on a health literacy framework, while the control group received the standard curriculum. Health literacy and weight-control dietary supplement use behaviors were assessed at baseline, immediately post-intervention, and at one-month follow-up using self-administered questionnaires. Intervention effects were analyzed using two-way mixed-design repeated-measures ANOVA. The intervention group showed significantly greater improvements in health literacy and safer weight-control dietary supplement use behaviors than the control group at both post-intervention and one-month follow-up (p < 0.001). Large effect sizes were observed for health literacy (partial η² = 0.864) and supplement-use behavior (partial η² = 0.928). These findings suggest that health literacy-based interventions may be an effective approach to strengthening female vocational students’ capacity to make safer decisions regarding weight-control dietary supplements and may inform health promotion initiatives for this population.