Regulatory T cells (Tregs) are a subpopulation of CD4+ T cells that are essential for maintaining the homeostasis of the immune system, limiting self-reactivity and excessive immune responses against foreign antigens. In cancer, infiltrated Tregs inhibit the effector lymphocytes and create a favorable environment for the growth of the tumor. Although Tregs mediate immunosuppression through multiple, non-redundant, cell-contact dependent and independent mechanisms, a growing body of evidence suggests an important role for the CD39–CD73–adenosine pathway. CD39 ectonucleotidase is the rate-limiting enzyme of a cascade leading to the generation of suppressive adenosine that alters CD4 and CD8 T cell and natural killer cell antitumor activities. Here, we review the recent literature supporting CD39 as a promising therapeutic target in oncology. In vitro and in vivo experiments involving knockout models and surrogate inhibitors of CD39 provide evidence in support of the anticancer activity of CD39 inhibition and predict a favorable safety profile for CD39 inhibitory compounds. In addition, we report the ongoing development of CD39-blocking monoclonal antibodies as potential anticancer drugs. Indeed, CD39 antagonistic antibodies could represent novel therapeutic tools for selectively inhibiting Treg function without depletion, a major limitation of current Treg-targeting strategies.