Non‐steroidal anti‐inflammatory drugs (NSAIDs) exert their effects primarily by inhibiting the activity of cyclooxygenase (COX), thus suppressing prostaglandin synthesis. Some NSAIDs are known to perform functions other than pain control, such as suppressing tumour cell growth, independent of their COX‐inhibiting activity. To identify NSAIDs with COX‐independent activity, we examined various NSAIDs for their ability to inhibit osteoblastic differentiation using the mouse pre‐osteoblast cell line MC3T3‐E1. Only celecoxib and valdecoxib strongly inhibited osteoblastic differentiation, and this effect was not correlated with COX‐inhibiting activity. Moreover, 2,5‐dimethyl (DM)‐celecoxib, a celecoxib analogue that does not inhibit COX activity, also inhibited osteoblastic differentiation. Celecoxib and DM‐celecoxib inhibited osteoblastic differentiation induced by bone morphogenetic protein (BMP)‐2 in C2C12 mouse myoblast cell line. Although celecoxib suppresses the growth of some tumour cells, the viability and proliferation of MC3T3‐E1 cells were not affected by celecoxib or DM‐celecoxib. Instead, celecoxib and DM‐celecoxib suppressed BMP‐2‐induced phosphorylation of Smad1/5, a major downstream target of BMP receptor. Although it is well known that COX plays important roles in osteoblastic differentiation, these results suggest that some NSAIDs, such as celecoxib, have targets other than COX and regulate phospho‐dependent intracellular signalling, thereby modifying bone remodelling.