Angiogenesis has been related with the expression of a water channel protein, Aquaporin-1 (AQP1), widely expressed in vascular endothelia where it increases plasma membrane water permeability and facilitates cell migration. We here hypothesized that AQP1 knockdown (KD) by RNA interference would affect the formation of new vessels and therefore the tumor growth. In vivo experiments were performed by intratumoral injection of lipid-formulated AQP1 specific siRNAs together with scrambled siRNAs as control on a well established mouse model of melanoma. Results showed that AQP1 specific siRNAs significantly reduced the tumor growth compared with the scrambled ones. AQP1 and Factor VIII expression levels were measured by Western blot. AQP1 interference induced a 7-fold reduction of AQP1 and a parallel 2.5-fold reduction of the endothelia marker Factor VIII, indicating a reduction of the number of vessels associated with AQP1 KD. Moreover, AQP1 immunofluorescence analysis showed a weak staining in AQP1 KD melanoma vessels whose diameter and number was significantly reduced. The results indicate that i) AQP1 has an important role in tumor angiogenesis, ii) AQP1 siRNA can efficiently target and inhibit angiogenesis and tumor growth when locally delivered, iii) RNA interference can be considered a new therapeutic approach for the inhibition of tumor growth.