Recently, studies on the use of polymer nanomaterial composites as pour-point depressants (PPD) have drawn much attention, but the crystallization properties and improved rheological performance of waxy crude oils using nanoclay-based composite PPDs have rarely been reported. In this paper, montmorillonite (Mnt) was first organically modified using octadecyltrimethylammonium chloride (C21H46NCl, or stearyltrimethylammonium chloride) in aqueous solution. Then, the organically modified Mnt (OMnt) material was dispersed into a polyoctadecylacrylate (POA) matrix to prepare a POA/OMnt composite PPD by melt blending. The composition, structure, and morphology of Mnt, OMnt, and the POA/OMnt composite PPDs were investigated. The results showed that the OMnt and POA were compatible and that the OMnt was exfoliated into several sheets in the POA matrix. Subsequently, the isothermal crystallization kinetics of the POA/OMnt composite PPDs showed that small amounts of OMnt had a dramatic impact on POA chain motion during crystallization and facilitated POA crystallization. After it was added to a waxy crude oil, the POA/OMnt composite PPDs produced better rheological properties and performance than identical concentrations of the neat POA. The POA/OMnt composite PPDs can act as wax nucleation sites for wax molecule precipitation and result in larger and more compact wax crystal flocs, which adversely affect the formation of a wax crystal network and, thus, favor the improvement of waxy crude oil rheology.