Stereoselective synthesis of the C1′−C25′ fragment of symbiodinolide, which was obtained as a degraded product from symbiodinolide by alkaline hydrolysis, has been accomplished. The synthetic route features Kotsuki coupling and Julia−Kocienski olefination in the introduction of the side chains. This enantio- and stereoselective synthesis has established the absoluteconfiguration of the C1′−C25′ fragment.