Matrix metalloproteinase 9 (MMP-9) has a key role in many biological processes, and while it is crucial for a normal immune response, excessive release of this enzyme can lead to severe tissue damage, as evidenced by proteolytic digestion and perforation of the cornea during infectious keratitis. Current medical management strategies for keratitis mostly focus on antibacterial effects, but largely neglect the role of excess MMP activity. Here, a cyclic tissue inhibitor of metalloproteinase (TIMP) peptidomimetic, which downregulated MMP-9 expression both at the mRNA and protein levels as well as MMP-9 activity in THP-1-derived macrophages, is reported. A similar downregulating effect could also be observed on α smooth muscle actin (αSMA) expression in fibroblasts. Furthermore, the TIMP peptidomimetic reduced Pseudomonas aeruginosa-induced MMP-9 activity in an ex vivo porcine infectious keratitis model and histological examinations demonstrated that a decrease of corneal thickness, associated with keratitis progression, was inhibited upon peptidomimetic treatment. The presented approach to reduce MMP-9 activity thus holds great potential to decrease corneal tissue damage and improve the clinical success of current treatment strategies for infectious keratitis.