To develop drugs to treat Alzheimer's disease (AD) on the basis of the amyloid cascade hypothesis, the amyloid-β (Aβ) aggregation inhibitory activities of 110 extracts from mushrooms were evaluated by thioflavin T (Th-T) assays. The MeOH extract of <i>Albatrellus yasudae</i> inhibited Aβ aggregation, and the bioactivity-guided fractionation of the extract afforded four novel meroterpenoids, named scutigeric acid (<b>1</b>), albatrelactone methyl ester (<b>2</b>), albatrelactone (<b>3</b>), and 10',11'-dihydroxygrifolic acid (<b>4</b>), together with two known compounds, grifolin (<b>5</b>) and grifolic acid (<b>6</b>). The structures of <b>1</b>-<b>4</b> were elucidated using NMR, MS, UV, IR, and induced ECD spectral data. The structure of <b>1</b> was determined as a methyl ester (<b>1a</b>) by 2D NMR spectroscopy. Th-T assays showed that compounds <b>1</b>-<b>4</b> and <b>1a</b> possessed inhibitory activities against Aβ aggregation, with IC<sub>50</sub> values of 6.6, 40.7, 51.4, 53.3, and 50.3 μM, respectively. Notably, <b>1</b> possessed an inhibitory activity against Aβ aggregation comparable to that of myricetin as a positive control. Moreover, <b>1</b>-<b>6</b> exhibited inhibitory activities against BACE1, with IC<sub>50</sub> values of 1.6, 10.9, 10.5, 34.4, 6.1, and 1.4 μM, respectively.