Background <p>Beta-cell function is a critical parameter for diagnosing and managing Type 2 diabetes mellitus (T2DM). Progressive β-cell failure is a key driver of disease progression and long-term complications in T2DM. While aerobic exercise has been shown to improve glycemic control, the specific effect of resistance training (RT) on β-cell function remains less explored. The aim of this review is to investigate whether RT improves β-cell function in people living with prediabetes or T2DM compared to control conditions, and to estimate the magnitude of this effect.</p> Methods <p>This systematic review and meta-analysis followed PRISMA 2020 guidelines. Six controlled trials involving a total of 581 participants were included. The sample consisted of adults with prediabetes or T2DM participating in structured RT programs. Measures of β-cell function, including HOMA-β, HOMA2-β, and the disposition index (DI), were assessed. Fasting-based indices were pooled using random-effects models where sufficient data were available, and remaining studies were synthesized narratively. The Cochrane RoB 2 tool and ROBINS-I were used to assess the risk of bias, and the GRADE framework was used to assess the certainty of evidence.</p> Results <p>Meta-analysis of three RCTs showed that resistance training produced significant improvements in fasting β-cell function indices compared with non-exercise or control conditions (Standardized Mean Difference [SMD] = 0.56; 95% CI: 0.19–0.91, <i>p</i> = 0.002). Included interventions ranged from 8 to 52 weeks, with moderate-to-high intensity protocols (60–80% 1-RM). Narrative synthesis of all six trials indicated favorable changes in fasting glucose and HbA1c, though not all comparisons reached statistical significance. Overall heterogeneity was moderate (I² = 34.6%).</p> Conclusions <p>The findings suggest that resistance training may enhance β-cell performance and cardiometabolic markers in adults with prediabetes or T2DM, with moderate-certainty evidence from RCTs. These results support the inclusion of RT in routine exercise prescriptions for diabetes management. However, the limited number of available trials and reliance on fasting-based indices highlight the need for larger, well-controlled studies using dynamic measures of β-cell function.</p> Systematic review registration <p><a href="https://www.crd.york.ac.uk/PROSPERO/view/CRD420251137273">https://www.crd.york.ac.uk/PROSPERO/view/CRD420251137273</a>.</p> Clinical trial registration number <p>Not applicable.</p>