Discoidin domain receptor 1 (DDR1) is overexpressed in various tumors, such as triple-negative breast cancer (TNBC), and is rarely expressed in normal tissues. These characteristics make DDR1 a preferable target candidate for the construction of an antibody-drug conjugate (ADC) for targeted therapy. Here, we investigated the preparation and preclinical efficacy of DDR1-DX8951, an ADC that includes an anti-DDR1 monoclonal antibody conjugated to DX8951 by a cleavable Gly-Gly-Phe-Gly (GGFG) linker. The anti-DDR1 monoclonal antibody was coupled to DX8951 (i.e., DDR1-DX8951), producing the targeted therapy ADC. The antitumor activities of DDR1-DX8951 monotherapy or DDR1-DX8951 plus pembrolizumab were assessed in TNBC mouse models. DDR1-DX8951 can specifically target DDR1, be quickly internalized by TNBC cells, and reduce the viability of TNBC cells in vitro. The potent antitumor activity of DDR1-DX8951 was revealed in TNBC xenograft models. Importantly, our investigation demonstrated that DDR1-DX8951 plus pembrolizumab not only revealed the inhibitory efficacy on tumor growth and metastasis but also played an important role in improving the immunosuppressive tumor microenvironment (TME) of TNBC. Taken together, this investigation provides justification for large-sample studies to further assess the safety and efficacy of DDR1-DX8951 plus pembrolizumab for TNBC clinical trials.