Polycystic ovary syndrome (PCOS) is the most common female endocrine disorder, and it has two main pathological aspects: reproductive and metabolic. Overweight/obesity is a risk factor in terms of adverse effects during hormone stimulation, a reduced response to ovulation induction regimens, reduced success of IVF, and an increased risk of obstetric complications. To resolve this vicious cycle of pathological events, weight loss and lifestyle modifications are promising strategies. Among these possible approaches, the consumption of a very-low-calorie ketogenic diet (VLCKD) or Mediterranean diet (MD) represents a valid option. In our study, 84 obese/overweight PCOS patients were recruited to evaluate the effects induced by the VLCKD and MD on weight, hormonal, and metabolic parameters. BMI decreased significantly among the VLKCD patients compared to the MD patients (both presenting p values < 0.0001 at 90 and 120 days), and a significant reduction in body circumference was observed. At the same time, HOMA index values statistically decreased for the VLCKD patients compared to those on the MD (p value < 0.001 at 90 days and p value < 0.05 at 120 days), and this phenomenon was also observed for AFC at 90 and 120 days (both p values < 0.001) and AMH at 90 days (p value < 0.05). Interestingly, the ovarian hyperstimulation syndrome (OHSS) incidence was statistically lower in the VLKCD patients compared to the MD patients (p < 0.001). We state that these dietary regimes may improve anthropometric parameters (such as BMI) and women’s reproductive health, restore menstrual regularity, and reduce the risk of OHSS. Regarding the different nutritional therapies, the results suggest that the VLCKD is an optimal choice for entry into IVF, especially in terms of the time range in which these results are achieved.