This study was designed to investigate the antiproliferative effect of a novel naphthoquinone derivative, 2-undecylsulfonyl-5,8-dimethoxy-1,4-naphthoquinone (2-undecylsulfonyl-DMNQ), on platelet-derived growth factor (PDGF)-stimulated vascular smooth muscle cells (VSMCs) and examine the possible molecular mechanism of its antiproliferative action. 2-Undecylsulfonyl-DMNQ significantly inhibited PDGF-stimulated cell number and DNA synthesis, and arrested the PDGF-stimulated progression through G0/G1 to S phase of cell cycle supported by the suppression of pRb phosphorylation and cyclin D1/E, CDK2/4 and PCNA expressions. 2-Undecylsulfonyl-DMNQ dose-dependently inhibited the PDGF-stimulated phosphorylation of phospholipase Cγ (PLCγ), protein kinase B (Akt/PKB), signal transducers and activators of transcription 3 (STAT3) and extracellular signal-regulated kinase 1/2 (ERK 1/2). In addition, 2-undecylsulfonyl-DMNQ inhibited PDGF-induced PDGF receptor β (PDGF-Rβ) dimerization and the phosphorylation of Tyr(579/581), Tyr(716), Tyr(751) and Tyr(1021) in PDGF-Rβ. However, 2-undecylsulfonyl-DMNQ has no antiproliferative effect on epidermal growth factor (EGF)- or fetal bovine serum (FBS)-stimulated VSMCs. In conclusion, these findings suggest that the antiproliferative effects of 2-undecylsulfonyl-DMNQ on PDGF-stimulated VSMCs are due to the blockade of receptor dimerization and autophosphorylation on specific tyrosine residues of PDGF-Rβ, which resulted in the subsequent suppression of signaling cascades and a cell cycle arrest. Our observation may explain an important mechanism to block the integration of multiple signals generated by growth factor receptor activation for prevention of VSMC proliferation in cardiovascular diseases.