Bone is the most common distant metastatic site of lung cancer, and is particularly prone to osteolytic damage. Soluble factors secreted from bone marrow-derived cells and tumor cells contribute to the growth and metastasis of cancer cells, and enhance osteolytic damage. BMP9, as the most powerful osteogenetic factor of the bone morphogenetic protein (BMP) family, can regulate the development of various tumors. However, the effects and underlying mechanisms of BMP9 in regards to lung cancer and the bone metastatic microenvironment are poorly understood. Here, we determined the inhibitory effects of BMP9 on the proliferation and migration of lung adenocarcinoma A549 cells. When a co-culture system of A549 cells and bone marrow-derived cells (HS-5) was established, it was shown that HS-5 cells promoted the proliferation and migration of A549 cells, and metastasis and osteoclast-related factors IL-6 and IL-8 were increased in the A549 and HS-5 cells. However, BMP9 inhibited the proliferation and migration of the A549 cells in the bone microenvironment, and decreased the levels of IL-6 and IL-8. In addition, mitogen-activated protein kinase (MAPK/ERK) and nuclear factor-κB (NF-κB) signaling pathway may be involved in these effects.