Sodium humate was oxidized with nitric acid to obtain fulvic acid (FA), which was further quaternized to obtain quaternary fulvic acid (QFA). QFA-intercalated saponite (QFA-saponite) was prepared ultrasonically. Thermoplastic poly(lactic acid) (PLA)/quaternary fulvic acid-intercalated saponite nanocomposites were prepared by melt intercalation technique. The morphology and dispersion of QFA-saponite were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Mechanical properties, thermal stability and crystallization behavior of PLA/QFA-saponite nanocomposites were also tested. Results showed a predominantly flocculated structure and partially intercalated morphology for QFA-saponite. Mechanical testing and thermogravimetric analysis showed that the tensile strength, impact properties, and thermostability of PLA/QFA-saponite nanocomposites improved significantly compared to pure PLA. Differential scanning calorimetry results showed that crystallinity of PLA increased from 22.5 to 68.3 % on addition of QFA-saponite. Polarized optical microscopy showed QFA-saponite as a nucleating agent for PLA that enhanced its crystallization rate. Rotational rheological behaviors of PLA/QFA-saponite nanocomposites demonstrated that incorporation of QFA-saponite increased rigidity of the network structure in PLA matrix.