Angiogenesis is a key process in the growth and metastasis of a tumor. Disrupting this process is considered a promising treatment strategy. Therefore, a drug delivery system specifically aiming at angiogenic tumor endothelial cells was developed. Alpha v beta 3-integrins are overexpressed on actively proliferating endothelium and represent a possible target. For this, RGD-peptides with affinity for this integrin were coupled to the distal end of poly(ethylene glycol)-coated long-circulating liposomes (LCL) to obtain a stable long-circulating drug delivery system functioning as a platform for multivalent interaction with alpha v beta 3-integrins. The results show that cyclic RGD-peptide-modified LCL exhibited increased binding to endothelial cells in vitro. Moreover, intravital microscopy demonstrated a specific interaction of these liposomes with tumor vasculature, a characteristic not observed for LCL. RGD-LCL encapsulating doxorubicin inhibited tumor growth in a doxorubicin-insensitive murine C26 colon carcinoma model, whereas doxorubicin in LCL failed to decelerate tumor growth. In conclusion, coupling of RGD to LCL redirected these liposomes to angiogenic endothelial cells in vitro and in vivo. RGD-LCL containing doxorubicin showed superior efficacy over non-targeted LCL in inhibiting C26 doxorubicin-insensitive tumor outgrowth. Likely, these RGD-LCL-doxorubicin antitumor effects are brought about through direct effects on tumor endothelial cells.