Macroalgae, also known as seaweeds, are vital to aquatic environments. Seaweeds exhibit numerous bio-cellular activities, including anti-diabetic properties, due to their complex chemical components, together with primary and secondary metabolites. The physiological active compounds present in seaweeds could manage digestive effects and maintain intestinal permeability. Also, they regulate glucose metabolism and insulin sensitivity, control postprandial blood glucose levels, and oxidative stress, thus demonstrating anti-diabetic properties. Bioactive compounds from seaweed have been characterised, and their actions have been partially elucidated. The anti-diabetic advantages of seaweeds have been proved by various in vitro, in vivo, and human studies. While oral seaweed intake remains common in many countries, additional research on the bioactivities and possible adverse effects of functional chemicals in edible seaweeds is essential for developing seaweed-based anti-diabetic medications. The main objective of this chapter is to give readers a basic understanding of the nutritional and anti-diabetic properties of macroalgae. The forthcoming chapters of this book include more comprehensive details about the mechanism of action and research studies in this area.