Background: Although a cholesterol-lowering diet and the addition of plant sterols and stanols are suggested for the lipid management of children and adults with familial hypercholesterolemia, there is limited evidence evaluating such interventions in this population. Objectives: To investigate the impact of cholesterol-lowering diet and other dietary interventions on the incidence or mortality of cardiovascular disease and lipid profile of patients with familial hypercholesterolemia. Search methods: Relevant trials were identified by searching US National Library of Medicine National Institutes of Health Metabolism Trials Register and clinicaltrials.gov.gr using the following terms: diet, dietary, plant sterols, stanols, omega-3 fatty acids, fiber and familial hypercholesterolemia. Selection criteria: Randomized controlled trials evaluating the effect of cholesterol-lowering diet or other dietary interventions in children and adults with familial hypercholesterolemia were included. Data collection and analysis: Two authors independently assessed the eligibility of the included trials and their bias risk and extracted the data which was independently verified by other colleagues. Results: A total of 17 trials were finally included, with a total of 376 participants across 8 comparison groups. The included trials had either a low or unclear bias risk for most of the assessed risk parameters. Cardiovascular incidence or mortality were not evaluated in any of the included trials. Among the planned comparisons regarding patients’ lipidemic profile, a significant difference was noticed for the following comparisons and outcomes: omega-3 fatty acids reduced triglycerides (mean difference (MD): −0.27 mmol/L, 95% confidence interval (CI): −0.47 to −0.07, p < 0.01) when compared with placebo. A non-significant trend towards a reduction in subjects’ total cholesterol (MD: −0.34, 95% CI: −0.68 to 0, mmol/L, p = 0.05) and low-density lipoprotein cholesterol (MD: −0.31, 95% CI: −0.61 to 0, mmol/L, p = 0.05) was noticed. In comparison with cholesterol-lowering diet, the additional consumption of plant stanols decreased total cholesterol (MD: −0.62 mmol/L, 95% CI: −1.13 to −0.11, p = 0.02) and low-density lipoprotein cholesterol (MD: −0.58 mmol/L, 95% CI: −1.08 to −0.09, p = 0.02). The same was by plant sterols (MD: −0.46 mmol/L, 95% CI: −0.76 to −0.17, p < 0.01 for cholesterol and MD: −0.45 mmol/L, 95% CI: −0.74 to −0.16, p < 0.01 for low-density lipoprotein cholesterol). No heterogeneity was noticed among the studies included in these analyses. Conclusions: Available trials confirm that the addition of plant sterols or stanols has a cholesterol-lowering effect on such individuals. On the other hand, supplementation with omega-3 fatty acids effectively reduces triglycerides and might have a role in lowering the cholesterol of patients with familial hypercholesterolemia. Additional studies are needed to investigate the efficacy of cholesterol-lowering diet or the addition of soya protein and dietary fibers to a cholesterol-lowering diet in patients with familial hypercholesterolemia.