<p>Matrix metalloproteinase-9 (MMP-9) plays a crucial role in cancer cell invasion, metastasis, and tumor growth. However, the precise and sensitive detection of MMP-9 remains a significant challenge, impeding a deeper understanding of its multifaceted roles in cancer progression. This study developed a novel surface-enhanced Raman scattering (SERS) sensing strategy based on DNA bending for ultrasensitive, highly selective detection of MMP-9. In the sensing system, MMP-9 aptamers and methylene blue–labeled complementary DNA were co-modified on a magnetic/plasmonic bead to fabricate a SERS probe. When target MMP-9 was present, the aptamer bound to the target and detached from the SERS nanoprobe, and the complementary DNA chain modified with methylene blue, due to its bending and the resultant closer proximity to the SERS nanoprobe surface, led to enhanced SERS signals. Meanwhile, cell experiments have confirmed that intensified hypoxia can directly lead to a significant increase in MMP-9 expression. Due to the high affinity between the aptamer and the target, this sensing system has high selectivity. The detection limit arrives at 3.67&#xa0;ng/mL. We applied this sensor to determine MMP-9 in patient sera and obtained consistent results with the standard method. This strategy is expected to develop into a new method for diagnosing specific cancers related to MMP-9 and for evaluating the occurrence, development, and extent of cancer metastasis.</p> Graphical abstract <p></p>