Objectives Rat dental follicle cells (rDFCs) function as precursor cells of periodontal tissues. Bone morphogenetic protein (BMP9) plays an important role in proliferation and differentiation. Tumour necrosis factor‐alpha (TNF‐alpha) is an important contributor to bone resorption. Wnt canonical pathway can be inhibited by Dickkopf 1 (DKK1). The aim of the study was to enhance the osteogenesis of BMP9 treated rDFCs in an inflammatory environment and elucidate the mechanism. Materials and Methods rDFCs were infected by adenoviruses expressing BMP9 (adBMP9). Expression levels of proteins and genes were measured by Western blotting and qPCR. The effect on osteogenesis was evaluated by measuring the activity of alkaline phosphatase (ALP), observation of Alizarin Red S and haematoxylin and eosin staining. Results TNF‐alpha activated the canonical Wnt pathway and inhibited the non‐canonical pathway. DKK1 suppressed the canonical pathway and promoted the non‐canonical pathway. Addition of TNF‐alpha or DKK1 inhibited BMP9/Smad pathway. However, this inhibition was reduced by the addition of DKK1 with TNF‐alpha. Conclusions DKK1 reduces the inhibitory effects of TNF‐alpha in adBMP9‐infected‐rDFCs, possibly through interaction with the Smad signalling pathway and Wnt pathways. These findings may lead to a novel approach for the treatment of periodontitis‐related alveolar bone defects.