Background: Prolactin levels have been shown to influence metabolic outcomes, including insulin resistance. Metformin is known to be beneficial in polycystic ovary syndrome (PCOS) patients. PCOS women might react differently to metformin treatment depending on their baseline prolactin levels. Methods: In this retrospective study, the homeostasis model assessment for insulin resistance (HOMA-IR), prolactin, luteinizing hormone (LH), follicle-stimulating hormone (FSH), the LH:FSH ratio, and total testosterone and sex hormone-binding globulin (SHBG) were measured in 75 obese/overweight women with PCOS and insulin resistance before initiation of metformin treatment and after 6–8 months. Results: At baseline, HOMA-IR was inversely correlated to SHBG (r = −0.408; p 14.9 ng/mL. Conclusions: In overweight/obese, insulin-resistant PCOS women, lower baseline prolactin levels are associated with higher baseline HOMA-IR levels as well as with a better response to metformin treatment. More data are necessary to prove these observations in larger populations.