Background: Hypertrophy of the ligamentum flavum (HLF) is a common contributor to spinal stenosis which results in significant neurological impairments. Circular RNA (circRNA) circ_0003609 has been linked to HLF; however, the exact mechanism by which it causes this disease is unclear. Methods: Circ_0003609 expressions were regulated in HLF cells by overexpression vectors and RNA interference. Cell proliferation and fibrosis-related gene expression were checked by the Cell Counting Kit-8 (CCK-8) assay and western blotting. CircBank's prediction of the association between miR-155 and circ_0003609 was supported by a dual-luciferase reporter experiment. The function of the miR-155/sirtuin 1 (SIRT1) axis in controlling HLF fibrosis was further examined. Results: Overexpression of circ_0003609 suppressed HLF cell propagation and fibrosis compared to its silencing. It was found that circ_0003609 served as the sponge for miR-155 and that the circ_0003609/miR-155 axis controlled the fibrosis of HLF cells. It was found that circ_0003609 acted as a sponge for miR-155, regulating the fibrosis of HLF cells. Further, miR-155 targets SIRT1, and the miR-155/SIRT1 axis promotes HLF cell fibrosis. Conclusion: Circ_0003609 ameliorates hypertrophied ligamentum flavum (LF) by modulating the miR-155/SIRT1 axis, indicating a potential treatment approach for HLF.