This Letter describes the identification of potent antioxidant and anti-osteoporosis agents from the fruits of Prunus mume. From the methanol extract, a novel flavan dimer, characterized as 2β,3β-epoxy-5,7,4'-trihydroxyflavan-(4α→8)-epicatechin (1), was isolated along with five known flavonoids (2-6). Their structures were determined based on extensive spectroscopic analysis, including IR, HRESIMS, 1D- and 2D-NMR, and CD spectra. The antioxidant activities of compounds 1-6 were evaluated in terms of their peroxyl radical-scavenging (Trolox equivalent) and reducing capacities. All isolates showed potent peroxyl radical-scavenging and reducing activities at concentrations of 1-10 μM. Among them, compounds 1 and 2 were the most active at 1 μM. Anti-osteoporosis activities were investigated using both murine osteoblastic MC3T3-E1 cells and osteoclastic RAW 264.7 cells. Compounds 2, 3, and 6 significantly stimulated the differentiation of osteoblastic MC3T3-E1 cells to increase collagen synthesis or mineralization functions of osteoblasts. Compounds 1, 3, 4, and 6 significantly suppressed tartrate-resistant acid phosphatase (TRAP) activity in receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastic RAW 264.7 macrophage cells.