The hydrogenation of nitriles and reductive amination of carbonyl compounds with ammonia over heterogeneous transition metal catalysts represent an atom-economical and scalable route to access value-added primary amines. However, there are selectivity issues in that sequential reactions proceed to give secondary and tertiary amines. Here we report a composite of metallic cobalt and silica particles that exhibits high catalytic performance for the selective synthesis of primary amines via hydrogenation of nitriles and reductive amination, even under mild conditions. A mechanistic study revealed that the substrate nitrile and the product amine retarded the hydrogenation of secondary imine intermediates, which resulted in a selective reaction. On the basis of this knowledge, the selectivity for the primary amine was further improved by the addition of coordinating organic molecules, which provided new insights into the control of selectivity toward primary amines unlike that with classical methods using ammonia.