T regulatory cells (Tregs) are an important T cell population for immune tolerance, prevention of autoimmune diseases and inhibition of antitumor immunity. The tumor-promoting role played by Tregs in cancer has prompted numerous approaches to develop immunotherapeutics targeting Tregs. One approach to depletion of Treg cells is retargeting the highly potent cytotoxic activity of bacterial toxins. These agents capitalize on the well-characterized bacterial toxins, diphtheria toxin and Pseudomonas aeruginosa exotoxin A–both of which harbor membrane translocation domains and enzymatic domains that catalytically halt protein synthesis within intoxicated eukaryotic cells and act at picomolar or subpicomolar concentrations. In this review, we summarize the preclinical and clinical development of several Treg-depleting cancer immunotherapies based on these two bacterial toxins.