Polymeric bionanocomposites are versatile materials that have many practical applications. The wide use of these materials is mainly due to their structure and properties, which combine at nanometric scale, the functionality of biopolymers with the excellent stability of inorganic solids, such as the clay minerals family. Montmorillonite is the most widely employed clay mineral in the development of polymeric bionanocomposites. The resulting materials from the combination of biopolymers and montmorillonite clay can form different structures (layered, delaminated, and exfoliated phases) and morphologies (powder, films or beads), exhibiting new features and better chemical, mechanical, thermal, and gas barrier properties compared to the pristine matrices, owing to the synergistic effect between both organic and inorganic moieties. In this sense, this review addresses polymeric bionanocomposites originating from the interaction between montmorillonite and chitosan and cellulose biopolymers. The synthesis, characteristics, and main applications of polymeric bionanocomposites have been reported, and in some cases, the relationship between the formed structures and final properties of bionanocomposites is also highlighted.