Rising atmospheric CO₂ concentrations are impacting the global terrestrial biosphere through indirect climate effects and direct effects on plant performance. In tropical forests, long-term monitoring indicates a substantial CO₂-driven carbon sink. C₄-grass-dominated tropical and subtropical savannas contribute 30% of terrestrial net primary production, and yet equivalent long-term analyses of CO₂ responses are lacking. Here, we show a clear and consistent result across a meta-analysis of 70 CO₂-addition experiments and 32 years of in situ field observations from southern Africa: CO₂ fertilization of wild C₄ grasses is widespread in dry conditions. In experiments, grasses reduced stomatal conductance under higher CO₂, limiting water loss while increasing carbon gain. In the field, improved water use efficiency translated into increased C₄ grass biomass production across three decades of observations. Finally, simulations via the Community Land Model suggest that CO₂ fertilization of C₄ grass aboveground productivity may continue to increase under future conditions. Together, these results challenge the view that C₄ grasses are unresponsive to increasing CO₂, demonstrating instead that annual grass aboveground production in the field in Southern Africa has increased by 28% over three decades (a CO~2-~driven increase of 75.1 g m^-2 [95% CI: 74.5-75.8 g m^-2^] or 0.37 tons C ha^-1 of annual production). Whilst the fate of this carbon is uncertain (depending on feedbacks with fire, herbivory, and woody vegetation), effects on the global carbon cycle may be profound.