Oligosaccharides possess fascinating functions that are applicable in a variety of fields, such as agriculture. However, the selective synthesis of oligosaccharides, especially chitin‐oligosaccharides, has remained a challenge. Chitin‐oligosaccharides activate the plant immune system, enabling crops to withstand pathogens without harmful agrichemicals. Here, we demonstrate the conversion of chitin to chitin‐oligosaccharides using a carbon catalyst with weak acid sites and mechanical milling. The catalyst produces chitin‐oligosaccharides with up to 94 % selectivity in good yields. Monte‐Carlo simulations indicate that our system preferentially hydrolyzes larger chitin molecules over oligomers, thus providing the desired high selectivity. This unique kinetics is in contrast to the fact that typical catalytic systems rapidly hydrolyze oligomers to monomers. Unlike other materials carbons more strongly adsorb large polysaccharides than small oligomers, which is suitable for the selective synthesis of small oligosaccharides. An activated carbon catalyst having weak acid sites hydrolyzes chitin to chitin‐oligosaccharides selectively in the presence of mechanical milling. Our system preferentially cleaves large molecules over small oligomers, thus maximizing the selectivity and yield of chitin‐oligosaccharides. The product mixture is a promising plant biostimulant for environmentally friendly agriculture.