<p>Nitrogen (N) assimilation consumes carbon (C) in plants, and climate warming can alter soil N availability, thereby affecting plant N assimilation and associated C costs. However, the global C cost of N assimilation in terrestrial plants has long been unconstrained, and its warming response remains uncertain. Here we constructed a modelling framework of plant C costs for nitrate, ammonium and extractable organic N assimilation and assessed global C costs of plant N assimilation. Globally, the C costs for plant N assimilation were 208 ± 12 and 249 ± 15 TgC yr⁻¹ under the present-day and 2.0 °C warming scenarios, respectively, which exceed the C emissions caused by deforestation and degradation fires and are comparable to the forest C fixed by atmospheric N deposition. Under the warming scenario, the global C cost would increase by 47% (41 ± 19 TgC yr⁻¹), with a greater percentage increase at higher latitudes, due partly to enhanced contributions and C cost of soil inorganic N. We conclude that the C cost for N assimilation in terrestrial plants should be incorporated into global C budgets, while its positive response to warming would improve predictions of terrestrial C-cycle feedbacks to global warming.</p>