The omega-3 fatty acid docosahexaenoic acid (DHA) is essential for nervous system and retinal development and there is evidence to suggest that DHA deficiencies increase with normal aging. A triple-blind placebo-controlled randomized repeated-measures trial was conducted with 74 healthy participants, aged 45–77 years. Cognitive and visual acuity measures and plasma levels of DHA were determined at baseline and after 90 days of administration of either HiDHA® (Clover Corp., Sydney, NSW, Australia: 1000 mg of tuna oil; comprising 252 mg DHA, 60 mg EPA and 10 mg vitamin E) or placebo (1000 mg soybean oil). Ninety days of DHA supplementation was found to significantly raise both plasma DHA and total ω-3 plasma levels in the treatment group, as well as significantly lower total ω-6 levels. However, no significant effects of DHA supplementation on cognitive functioning were found. For participants with corrected vision, the group receiving DHA were found to have significantly better right eye visual acuity posttreatment in comparison with the placebo group (F(1,22) = 7.651; p = 0.011; partial η2 = 0.258).