The development of all-bio-based composites is one of the relevant aspects of pursuing a carbon-neutral economy. This study aims to explore the possibility to reinforce polylactic acid by the combination of cellulose and chitin nanofibers instead of a single reinforcement phase. Polylactic acid colloidal suspension, cellulose and chitin nanofiber suspensions were mixed using only water as mixing medium and subsequently dewatered to form paper-like sheets. Sheets were hot pressed to melt the polylactic acid and form nanocomposites. The combination of cellulose and chitin nanofiber composites delivered higher tensile properties than its counterparts reinforced with cellulose or chitin nanofibers alone. Cellulose and chitin appear to complement each other from the aspect of the formation of a rigid cellulose nanofiber percolated network, and chitin acting as a compatibilizer between hydrophobic polylactic acid and hydrophilic cellulose.