Hydrogen sulfide (H2S) is a critical signaling molecule that regulates many physiological and/or pathological processes. Modulation of H2S levels could have potential therapeutic value. In this work, we report the rational design, synthesis, and biological evaluation of a class of phosphonamidothioate-based H2S-releasing agents (i.e., H2S donors). A novel pH-dependent intramolecular cyclization was employed to promote H2S release from the donors. These water-soluble compounds showed slow, controllable, and pH-sensitive production of H2S in aqueous solutions. The donors also showed significant cytoprotective effects in cellular models of oxidative damage. Most importantly, the donors were found to exhibit potent cardioprotective effects in an in vivo murine model of myocardial ischemia-reperfusion (MI/R) injury through a H2S-related mechanism.