Cell-penetrating peptides (CPPs) offer new opportunities for the delivery of drugs as they overcome the impermeability of the plasma membrane, and can be readily modified to confer tissue-specific targeting. We have discovered an entirely new class of cell penetrating peptide represented by a 7-amino acid peptide designated “Xentry” derived from an N-terminal region of the X-protein of the hepatitis B virus. Xentry permeates adherent cells using syndecans as a portal for entry, but is uniquely restricted from entering certain non-adherent cells, such as resting blood cells. Intravenous injection of Xentry alone or conjugated to beta-galactosidase led to its delivery to most tissues in mice, except circulating blood cells. Thus, it may have a therapeutic advantage as it is not taken up and diluted by blood cells. It has a predilection for uptake by epithelia, becoming concentrated in the epithelia of major organs, including the epithelial lining of the lungs and intestine. Xentry is able to deliver a variety of different biologically active cargo types to cells including peptides, proteins, antibodies, and siRNA. Anti-B-raf antibodies and siRNA linked to Xentry were capable of killing B-raf-dependent melanoma cells. Xentry represents a new class of CPP with properties that are potentially advantageous for life science and therapeutic applications.