The glycogen-synthase-kinase 3 (GSK-3) is an important target in drug discovery. This enzyme is involved in the signaling pathways of type 2 diabetes, neurological disorders, cancer, and other diseases. Therefore, inhibitors of GSK-3 are promising drug candidates for the treatment of a broad range of diseases. Here we report pannorin, alternariol, and alternariol-9-methylether to be promising inhibitors of the isoform GSK-3β showing sub-μM IC₅₀ values. The in vitro inhibition is in the range of the known highly active GSK-3β inhibitor TDZD-8. Compounds have a highly oxygenated benzocoumarin core structure in common, which suggests that this may be a new structural feature for efficient GSK-3β inhibition.