Secretory leukocyte protease inhibitor (SLPI) is a small, secreted protein that positively correlates with and belongs to the protein family of a common biomarker of ovarian cancer, HE4. SLPI is characterized by its immunoregulatory role in normal conditions, but its role in ovarian cancer is unclear. A single-cell RNA-sequencing analysis of the communication between cancer cells and immune cells in 34 high-grade serous cancers showed that SLPI interacts with CD4 receptors on immune cells, but the outcomes of this communication are not known. In this study, we aimed to identify the pathways regulating the expression of SLPI and determine its actions on CD4+ immune cells. Using an RNA-sequencing dataset of genetically defined mouse ovarian cancer cell lines, we analyzed and compared the baseline levels of SLPI to non-cancerous cell lines. The level of expression of SLPI was highly variable, and notably increased in cells with an activating KRAS mutation. By blocking the PI3K/AKT or MEK/ERK pathways using selective inhibitors, we determined that the level of SLPI expression decreases with the blockade of the MEK/ERK pathway, but not the PI3K/AKT pathway. This was confirmed by comparing ERK activity in ID8 (low SLPI expression) and STOSE (high SLPI expression), in which the latter had more phospho-ERK compared to ID8. To investigate the immune function of SLPI, CD4+ T cells from mouse spleens and lymph nodes were treated with recombinant SLPI, with or without concurrent activation with anti-CD3/CD28. In a dose-dependent manner, recombinant SLPI decreased the proliferation of CD4+ T cells and increased the expression of some immunosuppressive markers PD-1, PD-L1, and LAG3. Mouse ovarian cancer cells with overexpression or knockout of SLPI have been generated to determine its role in tumor progression. In conclusion, SLPI could be a good target to improve the immune response within the tumor microenvironment by blocking the immunosuppressive phenotype of cancer cells.