SCOPE: Limited information exists on how the relationship between dietary intake of fat and fatty acids in erythrocytes and plasma is modulated by polymorphisms in the FADS gene cluster. We examined gene‐diet interaction of total marine PUFA intake with a known gene encoding Δ‐5 desaturase enzyme (FADS1) variant (rs174550) for fatty acids in erythrocyte membranes and plasma phospholipids (PL), cholesteryl esters (CE), and triglycerides (TG). METHODS AND RESULTS: In this cross‐sectional study, fatty acid compositions were measured using GC, and total intake of polyunsaturated fat from fish and fish oil was estimated using a food frequency questionnaire in a subsample (n = 962) of the Metabolic Syndrome in Men Study. We found nominally significant gene‐diet interactions for eicosapentaenoic acid (EPA, 20:5n‐3) in erythrocytes (pᵢₙₜₑᵣₐcₜᵢₒₙ = 0.032) and for EPA in plasma PL (pᵢₙₜₑᵣₐcₜᵢₒₙ = 0.062), CE (pᵢₙₜₑᵣₐcₜᵢₒₙ = 0.035), and TG (pᵢₙₜₑᵣₐcₜᵢₒₙ = 0.035), as well as for docosapentaenoic acid (22:5n‐3) in PL (pᵢₙₜₑᵣₐcₜᵢₒₙ = 0.007). After excluding omega‐3 supplement users, we found a significant gene‐diet interaction for EPA in erythrocytes (pᵢₙₜₑᵣₐcₜᵢₒₙ < 0.003). In a separate cohort of the Kuopio Obesity Surgery Study, the same locus was strongly associated with hepatic mRNA expression of FADS1 (p = 1.5 × 10⁻¹⁰). CONCLUSION: FADS1 variants may modulate the relationship between marine fatty acid intake and circulating levels of long‐chain omega‐3 fatty acids.