CXCR4 is a coreceptor of HIV-1 infection in host cells. Through a photocrosslinking study to identify receptors involved in internalization of oligoarginine cell-penetrating peptides (CPPs), we found that CXCR4 serves as a receptor that stimulates macropinocytic uptake of the arginine 12-mer peptide (R12) but not of the 8-mer. We also found that stimulating CXCR4 with its intrinsic ligands, stromal cell-derived factor 1α and HIV-1 envelope glycoprotein 120, induced macropinocytosis. R12 had activity to prevent viral infection for HIV-1(IIIB), a subtype of HIV-1 that uses CXCR4 as a coreceptor for entry into susceptible cells, whereas the addition of a macropinocytosis inhibitor, dimethylamiloride, resulted in enhancement of viral infection. The present study shows that CXCR4 triggers macropinocytosis, which may have implications for the cellular uptake of oligoarginine CPPs and internalization of HIV.