The marine ecosystem is considered a potentially mysterious reservoir of biodiversity consisting of a broad spectrum of bioactive constituents, whose potential is still under research for pharmaceuticals, nutraceuticals, and cosmeceuticals. The prevalence of obesity has increased and is a health concern worldwide. Due to the concerns regarding synthetic anti-obesity treatments, nowadays natural products have become a trend. Marine algae, especially seaweeds, are a promising source of anti-obesity agents. Marine algae, particularly species like Laminaria, Undaria pinnatifida, and Ecklonia cava, are rich in bioactive compounds such as fucoxanthin, alginates, and polyphenols. These compounds exhibit significant anti-obesity and anti-diabetic properties, primarily through mechanisms like enhanced lipid metabolism, improved insulin sensitivity, and appetite suppression. Fucoxanthin, a carotenoid found in brown algae, has been shown to stimulate thermogenesis, thereby promoting fat loss and reducing body weight. Additionally, alginates have been demonstrated to decrease fat absorption and improve satiety, leading to reduced calorie intake. Polyphenols from marine algae possess strong antioxidant properties, which help mitigate oxidative stress and inflammation, both of which are critical in the pathogenesis of obesity-related diabetes. Clinical and preclinical studies indicate that marine algae can improve lipid profiles, reduce body fat, and enhance glucose metabolism, making them a potent natural intervention for managing obesity and preventing type 2 diabetes. These findings underscore the potential of marine algae as a functional food ingredient, although further research is needed to establish standardized dosages and long-term safety. Studies demonstrate that incorporating algal supplements into the diet can suppress appetite, lower body fat, and improve lipid profiles, offering a multifaceted approach to tackling obesity and diabetes. The findings underscore algae’s role as a promising, natural intervention for metabolic health, although further research is necessary to establish standardized dosages and long-term effects.