The oily compounds, which combat inflammation, appear to lower blood levels of a protein linked to Alzheimer's, scientists have found.
Researchers in the US studied 1219 people over the age of 65 who were free of dementia.
Participants were asked questions about their diet, and had their blood tested for beta-amyloid.
The protein clumps together in the brains of people with Alzheimer's and is one of the hallmarks of the disease.
Blood beta-amyloid levels were found to lower with greater consumption of omega-3 fatty acid.
A daily intake of one gram of omega-3 above the average amount consumed by the study participants was associated with a 20% to 30% reduction in beta-amyloid.
Scientists believe the levels of the protein in the blood reflect those found in the brain, indicating a protective effect caused by consuming foods rich in omega-3.
Dr Nikolaos Scarmeas, from Columbia University Medical Centre in New York, said: "While it's not easy to measure the level of beta-amyloid deposits in the brain in this type of study, it is relatively easy to measure the levels of beta-amyloid in the blood, which, to a certain degree, relates to the level in the brain.
"Determining through further research whether omega-3 fatty acids or other nutrients relate to spinal fluid or brain beta-amyloid levels, or levels of other Alzheimer's disease-related proteins, can strengthen our confidence on beneficial effects of parts of our diet in preventing dementia."
The scientists, whose findings are reported in the journal Neurology, looked at 10 nutrients including saturated fatty acids, omega-3 and omega-6 fatty acids, mono-unsaturated fatty acid, vitamins E, C, B12 and D, folate and beta-carotene.
The link between omega-3 and blood beta-amyloid remained the same after adjusting for a range of potential influences, including possession of a version of the APOE gene known to increase Alzheimer's risk.
Omega-3 fatty acids are mostly found in oily fish such as salmon, herring and mackerel. Lower amounts can also be found in nuts, Brussels sprouts, kale and spinach, and vegetable oils.
