With the discovery of the important biological roles of hydrogen sulfide (H2S), therapeutic applications of this gas have attracted growing attention. For medical applications of H2S, an increasing number of H2S donors, compounds which release H2S under physiological conditions, have been developed to evaluate the therapeutic potential of H2S. However, these low molecular weight molecules have intrinsic pharmacokinetic problems including poorly-controlled tissue distribution and rapid elimination from the blood circulation which may result in adverse effects and low therapeutic efficiency for practical applications. To overcome these limitations, polymeric micelles, spherical supramolecular assemblies formed from amphiphilic block copolymers with a diameter of 10–100 nm, have been used to improve pharmacokinetics, bioavailability and stability of drugs. In view of these feature, we sought to develop a micellar H2S delivery system. Here we report on novel polymeric micelles carrying H2S-releasing anethole dithiolethione (ADT) moieties. We prepared diblock copolymers having a hydrophilic poly (ethylene glycol) block and a hydrophobic ADT-bearing H2S-releasing block using reversible addition-fragmentation transfer (RAFT) polymerization. The resulting polymers self-assembled into monodisperse micelles with a diameter of 40 nm according to dynamic light scattering (DLS). Further characterization by transmission electron microscopy (TEM) confirmed that the polymers formed spherical micelles with narrow size distributions. The in vitro experiments using murine macrophages showed that the micelles enhanced lipopolysaccharide-induced nuclear factor-κB (NF-κB) activation and proinflammatory cytokine production. MTT assay revealed that the micelles have much lower toxicity compared to the small H2S donor ADT. This micellar system may offer a safe and efficient delivery of H2S in immunotherapy.