We assessed pregnancy outcomes associated with metformin use prior to assisted reproductive technology (ART) in women with polycystic ovary syndrome (PCOS). We conducted a population-based cohort study including a total of 10,178 women with PCOS undergoing their first fresh embryo transfer cycle under the national health insurance program of South Korea between 2017 and 2021. Metformin use within 12 weeks prior to ovarian stimulation was compared with non-use. Propensity-score overlap weighting was used to balance covariates. Among the included women, 2,149 (21.1%) used metformin before ovarian stimulation. Metformin use did not improve or worsen ART outcomes, with comparable rates of oocyte retrieval (risk ratio [RR] 1.01, 95% confidence interval 1.00–1.02), embryo transfer (RR 1.00, 0.98–1.02), and clinical pregnancy (RR 1.05, 1.00–1.11), as well as no increased rates of spontaneous abortion (RR 1.01, 0.84–1.21), preterm birth (RR 0.99, 0.84–1.18), or live birth (RR 1.05, 0.99–1.12). Subgroup analyses suggested modest benefits among women aged ≥ 35 years, those undergoing a GnRH antagonist protocol, and those with ≥ 30 days of metformin use. In this large cohort study, metformin pretreatment before ART was not associated with any clinically meaningful benefits or harms in pregnancy outcomes among women with PCOS. However, subgroup findings indicate potential benefits in selected populations, underscoring the need for further targeted investigations.