Dementia is expected to affect an increasing number of patients with global aging populations. About 70% of all dementia is related to Alzheimer's disease (AD). Overaccumulation of amyloid-β protein (Aβ) in the brain forms senile plaques, one of the main features of neurodegeneration in AD. However, there are few drugs available to specifically inhibit senile plaque formation. Fucoidan, a sulfated polysaccharide derived from brown algae, has various bioactivities, such as anti-tumoral and anti-obesity effects. This study aimed to clarify the mechanism underlying the protective effect of fucoidan against Aβ-induced neurotoxicity in human neuroblastoma SH-SY5Y cells. Cell viability and Aβ-induced cytotoxicity were measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, calcein AM, and ethidium homodimer-1. Aβ-induced oxidative stress was evaluated through reactive oxygen species (ROS), cell membrane phospholipid peroxidation, mitochondrial ROS, and Mn-SOD, a mitochondrial radical scavenger. In addition, mitochondrial membrane permeability transition, and ATP concentration were evaluated. Fucoidan significantly improved Aβ-reduced cell viability. With respect to oxidative stress, Aβ exposure increased ROS, lipid peroxidation, and mitochondrial ROS, while fucoidan significantly suppressed these changes. Fucoidan also suppressed the decline in mitochondrial permeability transition and ATP caused by Aβ. Therefore, through its numerous antioxidant activities, fucoidan might have a neuroprotective role in preventing Aβ-induced neurotoxicity.