Abnormal aggregation of beta-amyloid (A beta) peptides is a major hallmark of Alzheimer's disease (AD). In spite of numerous attempts to prevent the beta-amyloidosis, no effective drugs for treating AD have been developed to date. Among many candidate chemicals, methylene blue (MB) has proved its therapeutic potential for AD in a number of in vitro and in vivo studies; but the result of recent clinical trials performed with MB and its derivative was negative. Here, with the aid of multiple photochemical analyses, we first report that photoexcited MB molecules can block A beta(42) aggregation in vitro. Furthermore, our in vivo study using Drosophila AD model demonstrates that photoexcited MB is highly effective in suppressing synaptic toxicity, resulting in a reduced damage to the neuromuscular junction (NMJ), an enhanced locomotion, and decreased vacuole in the brain. The hindrance effect is attributed to A beta(42) oxidation by singlet oxygen (O-1(2)) generated from photoexcited MB. Finally, we show that photoexcited MB possess a capability to disaggregate the pre-existing A beta(42) aggregates and reduce A beta-induced cytotoxicity. Our work suggests that light illumination can provide an opportunity to boost the efficacies of MB toward photodynamic therapy of AD in future.