We investigated and forecasted the impact of diabetes mellitus (DM) on tuberculosis (TB) epidemiology in Indonesia between 2020 and 2050. A recently-developed age-structured TB-DM dynamic mathematical model was utilized to assess the impact of DM on TB epidemiology. Model parameters were informed by systematic reviews and meta-analyses. Sensitivity and uncertainty analyses were conducted to assess robustness of predictions. The proportion of TB incident cases attributed to DM increased from 18.8% (95% uncertainty interval (UI): 12.6%–24.3%) in 2020, to 20.9% (95% UI: 14.7%–27.1%) in 2030, and 25.8% (95% UI: 17.7%–32.2%) in 2050. The proportion of TB-related deaths attributed to DM increased from 24.3% (95% UI: 18.7%–29.1%) in 2020, to 27.7% (95% UI: 22.4%–32.4%) in 2030, and 34.3% (95% UI: 27.6%–38.0%) in 2050. Most of the impact of DM on TB transmission has risen because of faster progression to TB disease, increased risk of reinfection, and increased infectiousness, with higher bacterial loads. Sensitivity and uncertainty analyses affirmed the predictions. TB-DM synergy is projected to increase in Indonesia over the next three decades with DM becoming a major driver of TB incidence and deaths. Joint TB-DM management and programs could offer significant reductions in TB incidence and mortality, making post-2015 End TB targets more feasible.