NELL1 is a secretory osteogenic protein containing several structural motifs that suggest that it functions as an extracellular matrix component. To determine the mechanisms underlying NELL1-induced osteoblast differentiation, we examined the cell-adhesive activity of NELL1 using a series of recombinant NELL1 proteins. We demonstrated that NELL1 promoted osteoblastic cell adhesion through at least three cell-binding domains located in the C-terminal region of NELL1. Adhesion of cells to NELL1 was strongly inhibited by function-blocking antibodies against integrin α3 and β1 subunits, suggesting that osteoblastic cells adhered to NELL1 through integrin α3β1. Further, focal adhesion kinase activation is involved in NELL1 signaling.