Protozoan parasites contain negatively charged polymers of a few up to several hundreds of phosphate residues. In other organisms, these poly-phosphate (polyP) chains serve as an energy source and phosphate reservoir, and have been implicated in adaptation to stress and virulence of pathogenic organisms. In this study, we confirmed first that the polyP polymerase vacuolar transporter chaperone 4 ( <i>VTC4</i> ) is responsible for polyP synthesis in <i>Leishmania</i> parasites. During <i>Leishmania</i> <i>in vitro</i> culture, polyP is accumulated in logarithmic growth phase and subsequently consumed once stationary phase is reached. However, polyP is not essential since VTC4-deficient ( <i>vtc4 ^-</i> ) <i>Leishmania</i> proliferated normally in culture and differentiated into infective metacyclic parasites and into intracellular and axenic amastigotes. In <i>in vivo</i> mouse infections, <i>L. major</i> <i>VTC4</i> knockout showed a delay in lesion formation but ultimately gave rise to strong pathology, although we were unable to restore virulence by complementation to confirm this phenotype. Knockdown of <i>VTC4</i> did not alter the course of <i>L. guyanensis</i> infections in mice, suggesting that polyP was not required for infection, or that very low levels of it suffice for lesion development. At higher temperatures, <i>Leishmania</i> promastigotes highly consumed polyP, and both knockdown or deletion of <i>VTC4</i> diminished parasite survival. Thus, although polyP was not essential in the life cycle of the parasite, our data suggests a role for polyP in increasing parasite survival at higher temperatures, a situation faced by the parasite when transmitted to humans.