Seaweeds are recognized as valuable bioresources for functional food products due to their content of bioactive compounds, such as carotenoids. Brown, red, and green seaweeds each possess distinct carotenoid profiles, making them promising candidates for use in nutraceuticals and dietary supplements. Fucoxanthin (Fx), a major carotenoid found predominantly in brown seaweeds—but not in vegetables or fruits—has garnered particular attention. Dietary Fx has been shown to reduce body weight and white adipose tissue mass in diabetic/obese KK-Ay mice by inducing uncoupling protein 1 expression and suppressing chronic inflammation in white adipose tissue. Moreover, it lowers blood glucose levels by activating glucose transporter 4 within the insulin-signaling pathway. Moreover, Fx has demonstrated preventive effects against nonalcoholic fatty liver disease and nonalcoholic steatohepatitis in mice fed a choline-deficient, L-amino acid-defined high-fat diet. These findings highlight Fx as a multifunctional carotenoid with significant potential for health-promoting applications.