Abstract: Fucoidans are sulphated polysaccharides that are primarily derived from brown algae and marine invertebrates. Fucoidans generally have complex structural characteristics, which contribute to their various pharmacological activities including anti-tumor, anti-atherosclerotic, anti-inflammatory, antioxidant, anti-coagulant and lipid-lowering effects. However, fucoidans are difficult to be absorbed due to their large molecular weight. Comparatively, low-molecular weight fucoidans and fucoidan-derived oligosaccharides are easier to be absorbed, and these molecules are demonstrated to have equal or even better bioactivities compared with the original fucoidans. Therefore, degradation of fucoidan has become a research hotspot in recent years. Currently, chemical degradation, physical degradation, and biodegradation have been used for degradation of fucoidans. However, biodegradation primarily relies on special enzymes that are hard to be prepared and commercialized at present. It is worth noting that chemical degradation is characterized by a simple preparation process, low cost, and easy for transformation and application. During the past decades, significant progress has been made in this field. This article reviews the advances in chemical degradation of fucoidans by focusing on the degradation conditions and mechanisms of actions. This article provides information valuable for chemists that are interested in preparation of fucoidan-derived oligosaccharides and low-molecular weight fucoidans, for pharmacologists that are interested in investigating the bioactivities of fucoidans with different molecular weight, and for manufacturers that are interested in applying low-molecular weight fucoidans and fucooligosaccharides as functional foods and/or drug additives.