Postnatal environmental changes markedly facilitate functional closure of the ductus arteriosus (DA). Vascular remodeling during both fetal and postnatal periods is essential for achieving permanent anatomical DA closure; however, molecular mechanisms driving postnatal DA remodeling have yet to be fully elucidated. Single-cell RNA sequencing (scRNA-seq) revealed that postnatal mouse smooth muscle cells (SMCs) formed a transcriptionally distinct cluster compared with fetal ductus arteriosus smooth muscle cells (DASMCs), whereas other cell types remained in the same cluster after birth, highlighting a critical role for SMCs in postnatal DA remodeling. Transcriptome analysis identified genes differentially expressed in postnatal DASMCs compared with the adjacent arteries, among which cyclooxygenase-2 (COX-2) exhibited the most robust induction. Exposure to hydrogen peroxide, simulating oxidative stress encountered after birth, significantly increased COX-2 mRNA and protein expression in DASMCs. Given that platelet adhesion is a postnatal event in the DA and platelets are a major source of thromboxane A2, we administered thromboxane A2 receptor agonist to DASMCs and found marked COX-2 upregulation. Lentiviral-based overexpression of COX-2 led to prostaglandin E2 (PGE2) production. PGE2 stimulation increased expression of Nr4a1 via PGE2 receptor EP4. Nr4a1 silencing inhibited DASMC proliferation. To assess the in vivo relevance of COX-2, we maternally administered a selective COX-2 inhibitor SC-236 and found impaired postnatal DA closure in mice. These data suggest that postnatal upregulation of COX-2 in DASMCs promotes anatomical closure, potentially involving Nr4a1; inhibition of COX-2 at the very early postnatal period may interfere with DA closure.