Ecklonia spp. is available during summer (June-September), making it a possible rotational brown seaweed feedstock for biogas production. Hence, biogas production of Ecklonia was tested in a pilot-scale (120 L volume) semi-continuous fixed-bed digester under thalassic (high salinity) condition - seeded with marine bacteria. The biogasification efficiency obtained for Ecklonia in terms of its theoretical methane yield (345.6 mL CH4/g VS) was 68.5%. This is higher than obtained in a batch digester (5.8%). The successful thalassic biogas production of Ecklonia may help secure the continuous operation and lower the biogas operation cost by directly using seawater as liquid substrate.