Background: Diaporisoindole E (SA8), an isoprenylisoindole alkaloids isolated from the mangrove endophytic fungus Diaporthe sp. SYSU-HQ3, was reported with anti-inflammatory activity in RAW264.7 cells. However, the effect of SA8 in bone metabolism is unknown. Purpose: The purpose of this study is to investigate the inhibitory effect of SA8 in RANKL-induced osteoclastogenesis and to explore its mechanism of action. Methods: Osteoclastogenesis was assayed by TRAP staining. Expression of osteoclast specific genes was evaluated by real time-PCR. The inhibition of phosphorylation of the protein was measured by western blot analysis. The transcription activity of NF-κB was conducted using luciferase reporter gene assays. Osteoblast differentiation was assayed by alkaline phosphatase and Alizarin Red staining. Results: SA8 significantly inhibited the osteoclast differentiation in a dose- and time-dependent manner, which is consistent with the suppression of osteoclast specific genes including TRAP, DC-stamp, NFATc1, MMP-9, and ATP6v0d2. Further study on the mechanism of action revealed that SA8 inhibited osteoclast differentiation by attenuating PI3K/AKT and MAPK but not through NF-κB signaling pathways. Moreover, SA8 also suppressed bone resorption activity in a hydroxyapatite-coated plate without affecting osteoblast differentiation in C3H10T1/2 using alkaline phosphatase and Alizarin Red staining. Conclusions: These findings suggest that SA8 (Diaporisoindole E) is the potential anti-osteoporosis agent. © 2020 Elsevier GmbH