Verubecestat, atabecestat, and elenbecestat are small-molecule BACE1 inhibitors. Based on their structures, we designed and synthesized a novel BACE1 inhibitor with a hydroxyproline-derived N-amidinopyrrolidine scaffold. The initially synthesized derivative 7a showed a weak but detectable inhibitory activity against recombinant BACE1, which suggested that this novel scaffold was a viable BACE1 inhibitor. To enhance its activity, 22 derivatives with various substituents on the terminal benzene rings of the two biphenyl groups were synthesized and evaluated. Structure-activity relationship studies showed that introducing a substituent at the meta position of the biphenyl group on the hydroxy terminal improved the activity, and we identified the highly active derivative 12f. In contrast, substituents at the para position of the biphenyl group on the carboxy terminal increased activity. Additionally, we investigated the absolute configuration of the substituted pyrrolidine ring, which showed that the (2S,4R)-derivative exhibited the highest activity. Docking simulations suggested that a bulkier substituent tended to be located in the S1 and S3 pockets and that the binding mode significantly changed depending on which biphenyl group the substituent was attached to. These results show that the new scaffold would be useful for further development of small-molecule BACE1 inhibitors.