Hydrogen sulfide (H2S) is a gasotransmitter that plays several roles in various tissues, including the cardiovascular system. Because it has been recently proposed to act as a mediator of angiogenesis progression, here we investigate the effects of H2S in a well-established model of tumor angiogenesis: endothelial cells obtained from human breast carcinoma (B-TECs). Ca2+ imaging and patch-clamp experiments reveal that acute perfusion with NaHS, a widely employed H2S donor, activates cytosolic calcium (Cac) increase, as well as potassium and nonselective cationic currents, in B-TECs. Stimulation with NaHS in the same concentration range (1nM–200μM) evoked Cac signals also in “normal” human microvascular endothelial cells (HMVECs), but the amplitude was significantly lower. Moreover, although NaHS failed to promote either migration or proliferation on HMVECs, B-TEC migration was enhanced at low-micromolar NaHS concentrations (1–10μM). Remarkably H2S mediates tumor proangiogenic signaling triggered by vascular endothelial growth factor (VEGF). B-TECs pretreated with dl-propargylglycine (5mM, 30min), an inhibitor of the H2S-producing enzyme cystathionine γ-lyase, showed drastically reduced migration and Cac signals induced by VEGF (20ng/ml). We conclude that H2S plays a role in proangiogenic signaling of tumor-derived but not normal human ECs. Furthermore the ability of this gasotransmitter to interfere with B-TEC responsiveness to VEGF suggests that it could be an interesting target for antiangiogenic strategies in tumor treatment.