Phlegmadine A (1), a Lycopodium alkaloid with a unique cyclobutane ring and featuring a complex tetracyclo[4.2.2.03,8.03,10]decane-bridged system, together with three biogenetically related known compounds, was isolated from the Phlegmariurus phlegmaria. The structures and absolute configurations of 1 and 2 were determined by NMR and single-crystal X-ray analysis. Among them, compound 2 exhibited noticeable protective effects for long-term potentiation impairment by corticosterone induced in mice. Moreover, we succeeded in the efficient synthesis of 1 from 3 by a biomimetic synthesis method.