Polycystic ovary syndrome (PCOS) is a prevalent endocrine and metabolic disorder. This study investigated the mitigating effects of the Antrodia cinnamomea polysaccharide (ACP) on a letrozole-induced PCOS rat model. Results demonstrated that ACP reduced obesity and ameliorated dyslipidemia in PCOS rats. Moreover, ACP restored estrous cycle regularity, suppressed polycystic ovarian changes, and regulated serum levels of sex hormones, SOD, and MDA. Furthermore, ACP increased the αdiversity and modulated the abundance of phyla (Bacteroidetes, Firmicutes, and Verrucomicrobia) and genera (Lactobacillus, Helicobacter, Akkermansia, Oscillospira, Coprococcus, Roseburia, Blautia, and Allobaculum) in the gut microbiota. ACP also restored compromised intestinal barriers by upregulating the expression of ZO1, Occludin, Claudin1, and Claudin7 in the colon. ACP mitigated ovarian fibrosis by preventing activation of the NLRP3 inflammasome, decreasing the expression of fibrotic markers (TGF-β1, collagen-I, α-SMA, and CTGF), and regulating four ovarian fibrosis-associated metabolomics pathways. Generally, dietary ACP effectively ameliorated clinical symptoms and inhibited ovarian fibrosis in PCOS rats.