Polyendocrine metabolic ovarian syndrome (PMOS) features intertwined reproductive, endocrine and metabolic defects, yet its initiating mechanisms remain incompletely understood. Here we show that an epigenetic mechanism driven by nicotinamide N-methyltransferase (NNMT) in granulosa cells contributes to disease progression in PMOS. We observe that Nnmt overexpression in mouse granulosa cells reduces histone H3 lysine 27 trimethylation (H3K27me3), leading to increased retinoic acid receptor beta (RARB) expression. RARB, in turn, amplifies ovarian androgen synthesis. The resulting androgen excess further suppresses H3K27me3, locking cells into a feedforward loop that sustains hyperandrogenism and disrupts follicle development. Pharmacological inhibition of NNMT restores H3K27me3, disrupts the RARB-androgen feedback circuit, and ameliorates both metabolic and hormonal phenotypes in PMOS models. Our findings suggest NNMT as a regulator of PMOS through metabolic and epigenetic reprogramming. The NNMT-H3K27me3-RARB-androgen axis emerges as a mechanistic nexus linking methionine and epigenetic dysregulation to PMOS pathogenesis and represents a tractable therapeutic target.