The beneficial effects of n-3 polyunsaturated fatty acids (PUFA) in reducing high blood triglyceride (TG) levels have been well demonstrated. This study aimed to investigate the effect of chia seeds on blood TG and its associated cardiometabolic factors in hypertriglyceridaemic individuals. This three-group randomised controlled trial compared the effects of a low-calorie diet (n = 22), a low-calorie diet with chia seeds (30 g/day, n = 22) or a low-calorie diet with concentrated fish oil (1.8 g/day of n-3 long-chain PUFAs, n = 22) in patients with hypertriglyceridaemia. Anthropometrics, fasting blood lipids, proprotein convertase subtilisin/kexin type 9, insulin, adiponectin, leptin and interleukin-6 levels were measured. After 8 weeks, the mean reduction in weight exhibited by the three groups was not statistically different (2.0, 2.7 and 2.8 kg, respectively, for the control, fish oil and chia seed groups). The plasma TG decreased in both the chia seed and fish oil groups in comparison to the control group (p = 0.001). However, no significant difference was observed between the chia seed and fish oil groups (change from baseline mean: 145.2 and 136.7 mg/dL for the chia seed and fish oil groups, respectively). The consumption of chia seeds was associated with a reduction in diastolic blood pressure (change from baseline mean: 8.4 mmHg) compared to the other two groups. No significant alterations were observed in the other blood biochemical factors between the three groups. In people with moderate hypertriglyceridaemia, a low-calorie diet with 30 g of chia seeds compared to fish oil supplements containing 1.8 g of long-chain PUFAs has a similar effect on reducing plasma TG levels, whereas it has a higher blood pressure-lowering effect.