Polycystic ovary syndrome (PCOS) is defined as a reproductive endocrine disease that results in a low-grade inflammatory and pro-oxidant state. Dietary factors, including n-3 fatty acids, may have a key role in improving metabolic disorders in PCOS patients. The present study aimed to investigate the influence of n-3 fatty acid supplementation on inflammatory and oxidative stress (OS) markers in patients with PCOS. A systematic literature search of Medline/PubMed, Cochrane Central Register of Controlled Trials, Scopus and Lilacs, until November 2019, was conducted. Randomised clinical trials that reported inflammatory and OS markers as endpoints in women with PCOS receiving n-3 fatty acid supplementation were included. The pooled estimates of the weighted mean differences (WMD) and the standard mean differences (SMD) were calculated. Random effects models were adopted to measure the pooled outcomes. Among the 323 studies retrieved, ten fulfilled the inclusion criteria for a meta-analysis. We founded a significant decrease in high-sensitivity C-reactive protein (hs-CRP) (SMD –0·29 (95 % CI –0·56, –0·02) mg/l) and an increase in adiponectin (WMD 1·42 (95 % CI 1·09, 1·76) ng/ml) concentrations in the intervention group when compared with the placebo group. No statistically significant results were found in the meta-analysis for visfatin, nitric oxide, GSH or malondialdehyde levels or total antioxidant capacity. The data suggest that supplementation of n-3 fatty acids could reduce the inflammatory state in women with PCOS, through a decrease in hs-CRP and an increase in adiponectin levels.