Background: Matrix metalloproteinase-9 (MMP-9) is a significant protease required for synaptic plasticity, learning, and memory. Yet, the role of MMP-9 in the occurrence and development of cognitive decline after ischemic stroke is not fully understood. In this study, we used clinical data experiments to further investigate whether MMP-9 and genetic polymorphism are associated with post-stroke cognitive impairment or dementia (PSCID). Materials and Methods: A total of 148 patients with PSCID confirmed by the Montreal Cognitive Assessment (MoCA) 3 months after onset (PSCID group) were included in the study. The MMP-9 rs3918242 polymorphisms were analyzed using polymerase chain reaction coupled with restriction fragment length polymorphism, and the serum level of MMP-9 was measured using enzyme-linked immunosorbent assay (ELISA). The same manipulations have been done on 169 ischemic stroke patients without cognitive impairment (NCI group) and 150 normal controls (NC group). Results: The expression level of serum MMP-9 in the PSCID group and NCI group was higher compared to the NC group, and the levels in the PSCID group were higher than that in the NCI group (all p 0.05). Diabetes mellitus, hyperhomocysteinemia, and increased serum MMP-9 levels were the main risk factors of cognitive impairment after ischemic stroke. The serum level of MMP-9 was negatively correlated with the MoCA score, including visual-spatial executive, naming, attention, language, and delayed recall. Genetic polymorphism showed that TC genotype with MMP-9 rs3918242 and CC genotype were associated with a significantly increased risk of PSCID; moreover, the TC genotype significantly increased the risk of cognitive impairment. In the TCCC genotype of MMP-9 rs3918242, diabetes mellitus and hyperhomocysteinemia were associated with the increased risk of PSCID; also, hyperhomocysteinemia could increase the risk of cognitive impairment. Conclusions: MMP-9 level and MMP-9 rs3918242 polymorphism have an important role in the occurrence and development of post-stroke cognitive impairment or dementia (PSCID).