Objective: Determine the effect of interleukin (IL)-15 on HTR-8/SVneo cells and a preeclampsia (PE) mouse model induced by LPS. Methods: Transwell and Annexin-V-FITC/PI assays were performed in HTR-8/SVneo cells transfected with IL-15 activation plasmid/siRNA prior to LPS treatment. Additionally, pregnant mice were injected with LPS and IL-15 siRNA followed by measurement of systolic blood pressure (SBP), urine protein, and serum NO. HE staining was used to observe the morphological changes of the placenta and kidney. Glycogen accumulation was detected using Best’s carmine. qRT-PCR, Western blotting, and ELISA were performed to detect mRNA and protein expression. Results: LPS increased IL-15 and IFN-γ expression in HTR-8/SVneo cells, and IL-15 positively regulated IFN-γ expression in LPS-induced HTR-8/SVneo cells. Moreover, LPS promoted apoptosis and reduced the invasion and migration of HTR-8/SVneo cells, which was, further, promoted by IL-15 overexpression but attenuated by IL-15 inhibition. Furthermore, LPS increased SBP and urine protein but decreased serum NO in mice, and these factors were reversed by IL-15 siRNA. Downregulation of IL-15 also mitigated kidney injury and improved pregnancy outcomes in LPS-induced PE mice. A significantly thicker junctional zone (JZ) and thinner labyrinth layer were found in placentas of PE mice treated with IL-15 siRNA, along with increased glycogen trophoblast cells in the JZ. Moreover, decreased IFN-γ and NKp46 were found in placentas of PE mice treated with IL-15 siRNA. Conclusion: IL-15 inhibition reduced cell apoptosis and increased the invasive and migratory abilities of LPS-induced HTR-8/SVneo cells, thereby alleviating the PE-like phenotype and improving pregnancy outcome.