Plant sterols are well-known natural lipid-lowering agents. The DESCO (Diet and plant sterols in the control of cholesterolemia) study was a single-center, randomized, double-blind, placebo-controlled, two-way crossover clinical trial designed to investigate the effect of a once-a-day ready-to-drink dietary supplement containing 2.5 g of phytosterols on the lipid profile, also in relation to the quality of the diet, in a cohort of 50 Italian individuals with polygenic hypercholesterolemia and low global cardiovascular risk. Eligible individuals were enrolled in a run-in period of 2 weeks. Then, participants who qualified for continuation in the study were randomly allocated (1:1) to a 3-week treatment with either phytosterols or placebo. After a 2-week washout period, enrolled individuals were crossed over to receive the alternative treatment. Dietary supplementation with phytosterols was associated with significant improvement in plasma levels of total cholesterol (TC; −11.8 ± 4.0 mg/dL, p = 0.016), low-density lipoprotein cholesterol (LDL-C; −7.8 ± 7.7 mg/dL, p = 0.021), and apolipoprotein B-100 (Apo B-100, −3.7 ± 4.1 mg/dL, p = 0.048) compared to baseline. The changes in TC and LDL-C were also significant compared to placebo, and greater adherence to the Mediterranean diet was significantly associated with greater reductions in LDL-C. Dietary supplementation with phytosterols was well tolerated and adherence to treatment was high. According to the findings of DESCO, the once-a-day ready-to-drink dietary supplement we tested is able to quickly and significantly decrease plasma levels of TC, LDL-C, and Apo B-100, with a greater effect in individuals more adhering to the Mediterranean dietary pattern.