Cytological features of placental site plaques in liquid-based cervicovaginal preparations have been seldom documented in the literature. We present a rare case of endocervical placental site plaque misinterpreted as a low-grade squamous intraepithelial lesion in a liquid-based cytological preparation. A 32-year-old woman with polycystic ovarian syndrome gave birth 7 months previously. After delivery, she was diagnosed with cervical low-grade squamous intraepithelial lesion during routine cytological examination. Cytologically, many atypical cells showed large hyperchromatic nuclei with irregular membranes. The perinuclear cytoplasmic clearing closely resembled koilocytosis. Histologically, the endocervix showed typical histological features of a placental site plaque. Immunohistochemically, the trophoblasts were positive for p63, CD10, and inhibin-α but negative for p16. Based on genotyping, both the cytological and biopsied specimens tested negative for human papillomavirus. We re-examined the liquid-based preparation cytology slides thoroughly and concluded that the atypical cells initially misinterpreted as low-grade squamous intraepithelial lesion were actually trophoblasts. Immunocytochemical staining revealed uniform cytoplasmic inhibin-α expression in the trophoblasts. In summary, we demonstrated that endocervical placental site plaques can mimic low-grade squamous intraepithelial lesions in liquid-based cytological preparations. Immunocytochemical staining results and negative results on human papillomavirus genotyping further support that atypical cells resembling koilocytes are trophoblasts obtained from the placental site plaque.