Seafood is a rich source not only of proteins, minerals, vitamins, micronutrients, and super-unsaturated fatty acids, but also a prime provider of umami taste from both free glutamate as well as free nucleotides that enter into a powerful taste synergy. Plant-based foods, apart from some ripe fruits, are in contrast poor in umami taste. Since humans are genetically primed to seek out foods with umami taste, the lack of umami in plants presents a real challenge when implementing a green transition in the diet away from meat towards a more plant-rich diet. In this paper, the umami potential of a range of different seafoods will be described, with a focus on fish, mussels, cephalopods, crustaceans, echinoderms, roe, and seaweed. Mention will also be made of seafood-based stocks, fermented sauces, and condiments. Gastronomic examples of using these seafoods for umamification of plant-based foods will be described. Insightful use of blue foods in the green cuisine may contribute to a sustainable, healthy, and nutritious plant-forward diet with less meat and without compromising taste.