Background <p>Niraparib (Nir) is a poly(ADP-ribose) polymerase inhibitor (PARPi) used in the maintenance treatment of platinum-sensitive ovarian cancer (OC) patients, regardless of homologous recombination deficiency status. Being overweight or obese increases the risk of developing both OC and diabetes. Given this overlap, understanding the effect of Nir on glycemia is particularly important; however, it remains poorly understood.</p> Methods <p>The study included 22 normoglycemic OC patients. Fasting glucose (FG) concentrations were measured before therapy and after the second, third, and fourth treatment cycle (a cycle is approximately 28 days). A linear mixed-effects model treating body mass index (BMI) as a continuous variable was applied for statistical calculations.</p> Results <p>Impaired fasting glucose (IFG) (5.6–6.9 mmol/L) was observed in 55% of patients at some point during the study, and in 27% throughout its duration. A significant interaction between BMI and time was detected (<i>p</i> = 0.0412), with the influence of BMI on Nir-caused hyperglycemia increasing as the study progressed.</p> Conclusions <p>Hyperglycemia appears to be an adverse effect of Nir. As there is some indication that BMI may influence this effect, glycemic control is recommended, particularly in patients with an elevated BMI.</p>