Enhancing the therapeutic efficacy of drugs for inflammatory diseases is of high demand. One possible approach is targeting drugs to the extracellular matrix of the inflamed area. Here, we target collagens in the matrix, which are inaccessible in most tissues yet are exposed to the bloodstream in the inflamed area because of vascular hyperpermeability. We conferred collagen affinity to anti–tumor necrosis factor-α (α-TNF) antibody by conjugating a collagen-binding peptide (CBP) derived from the sequence of decorin. CBP–α-TNF accumulated in the inflamed paw of the arthritis model, and arthritis development was significantly suppressed by treatment with CBP–α-TNF compared with the unmodified antibody. Similarly, CBP–anti-transforming growth factor-β (α–TGF-β) accumulated in the inflamed lung of pulmonary fibrosis model and significantly suppressed pulmonary fibrosis compared with the unmodified antibody. Together, collagen affinity enables the anticytokine antibodies to target arthritis and pulmonary fibrosis accompanied by inflammation, demonstrating a clinically translational approach to treat inflammatory diseases.