More than 50% of prostate cancer (PCa) patients have bone metastasis with osteoblastic lesions. MiR-18a-5p is associated with the development and metastasis of PCa, but it remains unclear whether it is involved in osteoblastic lesions. We first found that miR-18a-5p was highly expressed in the bone microenvironment of patients with PCa bone metastases. To address how miR-18a-5p affects PCa osteoblastic lesions, antagonizing miR-18a-5p in PCa cells or pre-osteoblasts inhibited osteoblast differentiation in vitro. Moreover, injection of PCa cells with miR-18a-5p inhibition improved bone biomechanical properties and bone mineral mass in vivo. Furthermore, miR-18a-5p was transferred to osteoblasts by exosomes derived from PCa cells and targeted the Hist1h2bc gene, resulting in Ctnnb1 up-regulation in the Wnt/β-catenin signaling pathway. Translationally, antagomir-18a-5p significantly improved bone biomechanical properties and alleviated sclerotic lesions from osteoblastic metastases in BALB/c nude mice. These data suggest that inhibition of exosome-delivered miR-18a-5p ameliorates PCa-induced osteoblastic lesions.