Polyunsaturated fatty acids (PUFAs) have been shown to exert a wide range of health benefits. In particular, n-3 long chain polyunsaturated fatty acids (n-3 LC-PUFAs), abundant in marine dietary products, play a major role in the prevention and treatment of obesity and obesity-related disorders, particularly type 2 diabetes, hypertension, and cardiovascular diseases. The etiology of obesity and its associated disorders involves multiple biochemical and physiological pathways; thus, prevention and treatment require action on multiple targets/organs. Recent research showed that n-3 PUFAs limit fatty acid availability in white adipose tissues, therefore preventing and/or controlling overweight and obesity. In addition, PUFAs are also involved in other processes including gene and cytokine expression, eicosanoid production, and gut microbial regulation, all of which can significantly improve obesity and obesity related diseases. Macroalgae (seaweeds) and microalgae are capable of de novo PUFAs synthesis and hence represent a vast and untapped PUFAs source, which may be more resourceful and sustainable than fish.