Poly(lactic acid) (PLA) has emerged as a promising biodegradable polymer for alternate petroleum-based plastics. However, its limited thermal stability and crystallinity have required the incorporation of reinforcing materials. Among them, cellulose nanofibers (CNFs) enhance the physical properties and sustainability of PLA due to their high specific surface area, excellent mechanical strength, and biodegradability. However, the intrinsic hydrophilic nature limits compatibility with PLA, leading to restricted improvement in properties. In this study, stearic acid (SA), an amphipathic material, was coated on the surface of CNF (SACNF) in an aqueous system without any organic solvents. SA reduced the aggregation of CNF due to hydrogen bonding, thereby enhancing its compatibility with PLA. Furthermore, the production efficiency of composites was improved by increasing the melt flow index due to the SA as a lubricant. SACNF/PLA composites exhibited a 41.7% increase in crystallinity compared to neat PLA, attributed to the nucleation effect of SACNF. The network structure of fillers improved the storage modulus to 3108 MPa and raised the heat deflection temperature to 133.8 °C. Moreover, the biodegradability was enhanced by 20% due to the biodegradable CNF. Consequently, we developed an aqueous process for producing SACNF and successfully fabricated composites with PLA.