Problem: Preeclampsia is a major cause of fetal and maternal mortality and morbidity. Disturbed fetal-maternal immune tolerance, and therewith memory T cells, might be involved in its etiology. This study aims to give insight into memory T-cell populations and its associated cytokines in the decidual layers in early-onset preeclampsia (EO-PE) and late-onset preeclampsia (LO-PE). Method of Study: Lymphocytes were isolated from the decidua parietalis and basalis from EO-PE (n = 6), LO-PE (n = 8) and healthy (n = 15) pregnancies. CD4+ and CD8+ central- (CCR7+), effector- (CCR7−), tissue resident- (CD103+), and regulatory- (Foxp3+) memory cell (CD45RO+) populations and their activation status (CD69+) were analyzed using flow cytometry. qRT-PCR analysis was performed on decidua parietalis and basalis biopsies to detect mRNA expression of interferon-gamma, interleukin-1B, IL2, IL6, IL7, IL8, IL10, IL15, and IL23. Results: CD4+ central-memory (CM) cell proportions were lower in the decidua parietalis in LO-PE (P <.0001) and EO-PE (P <.01) compared to healthy pregnancies. CD8+ memory (P <.05) and CD8+ CM (P <.01) cell proportions were also lower in the decidua parietalis in EO-PE compared to healthy pregnancies. This was accompanied by higher IL15 (P <.05) and IL23 (P <.05) and lower IL7 (P <.05) mRNA expression in decidua basalis biopsies from EO-PE compared to healthy pregnancies, analyzed by qPCR. Conclusion: In conclusion, decidual memory T-cell proportions, their activation status, and associated cytokines are altered in preeclampsia and might therefore be involved in fetal-maternal immune tolerance and the pathophysiology of preeclampsia.