Chemical depolymerization of chitosan was obtained in the solid state by means of gaseous HCl. This new method allowed us to simultaneously form the hydrochloric salt and saturate the hydration water with acid. The depolymerization was carried out by keeping the product at a given temperature for the desired time. The measurements of the molecular weight distributions demonstrated the ability to control the reaction and produce oligomers with chosen dimensions. This solid-state hydrolysis favors the presence of a four-fold distribution that can be related to the original morphology and crystallinity of the initial material. The effects of the hydrolysis on the crystallinity, the crystalline structure, and the supramolecular order of the obtained chitosans were also studied. Finally, the washing of the hydrolyzed products in concentrated alkaline or acidic media allowed us to eliminate the lowest DPs and thus to narrow the molecular weight distribution. In this case, the crystallinity was also increased up to values beyond 70%.