Mussel powder provides the long-chain omega-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from food-grade mussels unsuitable for fresh-market sale that would otherwise be discarded, offering an opportunity to reduce seafood waste Assessing omega-3 biomarker response and acceptability can inform the development of this underused food source as a commercial supplement. Therefore, tis study investigated whether mussel-powder supplementation increased the Omega-3 Index (O3I; erythrocyte EPA + DHA as a percentage of total fatty acids) and assessed consumer acceptability. In this six-week double-blind randomised controlled trial, 49 adults (27 females and 22 males; median age 24 years [range 21-42]; body mass index 23.2 ± 2.7 kg/m²) consumed four 1 g capsules/day providing either South Coast mussel powder (n = 26; approximately 186 mg/day EPA + DHA) or a sucrose control (n = 23). At baseline and follow-up, finger-prick dried-blood-spot samples were analysed for whole-blood fatty acids, with O3I estimated using a validated equation based on combined whole-blood EPA and DHA concentrations (r = 0.96).(1) A marine food-frequency questionnaire adapted from a previously validated tool assessed past-month oily fish, white fish and other seafood consumption frequency and typical portion size.(2) Standardised whole-body dual-energy X-ray absorptiometry after an overnight fast assessed body-composition. Post-trial adherence and consumer perceptions were assessed using returned capsule counts and an online survey, respectively. Group-by-time effects were estimated using linear or generalised linear mixed models, with Holm adjustment across prespecified outcomes. O3I did not change differentially between groups (control: 6.08 ± 1.16% to 6.05 ± 1.07%; mussel: 6.19 ± 1.21% to 6.01 ± 1.18%; p = 0.524). No group-by-time effects remained for whole-blood total omega-3, EPA or DHA (all, p ≥ 0.176). Self-reported seafood intake remained stable over time and did not differ between groups (all, p ≥ 0.114), while body-composition changes also did not differ between groups (all, p ≥ 0.515). Median capsule adherence was 82%, with no group difference (p = 0.489). General acceptability ratings did not differ between groups (all, p ≥ 0.084). Pooled results showed that most participants found the capsules easy to take (70%), acceptable in number (64%) and easy to incorporate into their routine (67%), while 53% were willing to continue taking them if commercially available. Unpleasant smell was reported more frequently with mussel powder than control (63% versus 0%; p < 0.007). Although this dose did not alter omega-3 biomarkers, adherence was high and the capsules were generally well accepted. These findings support the development of a commercial supplement made from mussels that would otherwise be discarded, although the dose and formulation may require modification.