To modulate T cell function for cancer therapy one challenge is to selectively attenuate regulatory but not conventional CD4+ T cell subsets (Treg and Tconv). In this study we show how a functional dichotomy in Class IA PI3K isoforms in these two subsets of CD4+ T cells be exploited to target Treg while leaving Tconv intact. Studies employing isoform-specific PI3K inhibitors and a PI3Kδ-deficient mouse strain revealed that PI3Kα and PI3Kβ were functionally redundant with PI3Kδ in Tconv. Conversely, PI3Kδ was functionally critical in Treg, acting there to control TCR signaling, cell proliferation and survival. Notably, in a murine model of lung cancer, co-administration of a PI3Kδ-specific inhibitor with a tumor-specific vaccine decreased numbers of suppressive Treg and increased numbers of vaccine-induced CD8 T-cells within the tumor microenvironment, eliciting potent anti-tumor efficacy. Overall, our results offer a mechanistic rationale to employ PI3Kδ inhibitors to selectively target Treg and improve cancer immunotherapy.