The selective reductive N-methylation and N,N-dimethylation of primary amines using CO₂ are some of the most significant challenges faced by organic chemists. Herein, we report the highly selective N,N′-diisopropylcarbodiimide-catalyzed methylation of primary amines using 1 atm CO₂ under metal-free conditions. Borane–piperazine and borane–trimethylamine complexes were used as reducing agents for the N-methylation and N,N-dimethylation of various aromatic primary amines, respectively, in the presence of CO₂. Mechanistic studies suggest that the selectivity of methylation is controlled by the steric effects of amines and boranes.