Iloprost, a stable prostaglandin I₂ (PGI₂) analog, can inhibit allergic inflammation in an ovalbumin (OVA)-induced asthma model via inhibition of airway dendritic cell (DC) function. However, the underlying mechanism of PGI₂ signaling-mediated immunosuppression remains unclear. This study explored whether iloprost-treated DCs can suppress inflammation by promoting antigen-specific regulatory T cell (Treg) differentiation through PGI₂-G-protein-coupled receptor (IP). We established an allergic lung inflammation model using a hydrogel biomaterial delivery system and observed that iloprost significantly suppressed OVA-induced Th2 lung inflammation and increased the frequency of OVA-specific Tregs in vivo. We further observed that iloprost-treated DCs displayed tolerogenic characteristics, including low inflammatory cytokine (IL-12, TNF-α, IL-6, IL-23) expression levels, high anti-inflammatory cytokine (IL-10) production, and a semimature phenotype. In addition, iloprost-treated DCs increased OVA-specific CD4⁺Foxp3⁺ T cell differentiation from naïve T cells in an IP-dependent pathway in vitro and in vivo. Blocking experiments showed that iloprost-treated DCs promoted Treg differentiation, at least in part, through programmed death ligand 1 (PD-L1), whereas iloprost-induced PD-L1 expression in DCs was through the IP receptor. Furthermore, iloprost treatment suppressed DC-mediated airway inflammation and increased the frequency of OVA-specific Tregs through PD-L1 in vivo. Taken together, these results show that PGI₂-IP signaling mediated by iloprost in DCs may lead to immune tolerance, suggesting that the PGI₂ analog has the potential to be applied therapeutically for tolerogenic DC immunotherapy in autoimmune diseases or allergic asthma.