ATF3 expression was regulated during osteoclast differentiation. Overexpression of ATF3 enhanced osteoclast differentiation and activity. Inhibition of ATF3 resulted in reduced osteoclastogenesis. ATF3 synergistically enhanced promoter activity of TRAP and NFATc1 by associating with NFAT and c-Fos. ATF3 modulated calcium-signaling cascade during osteoclastogenesis.