Background: Placenta accreta spectrum (PAS) can easily lead to life-threatening hemorrhage. However, the association between placental thickness (PT) and massive bleeding remains unclear. Thus, this study investigated the association between PT and massive bleeding to determine which patients with suspected PAS and placenta previa were more likely to experience intraoperative hemorrhage. Methods: This retrospective cohort study was conducted between January 2018 and December 2020 at a general tertiary care hospital in Chongqing, China. Covariates included demographic, clinical, and ultrasonographic characteristics. Logistic regression analysis was used to explore the association between PT and massive bleeding. A sensitivity analysis was conducted by detecting trends in the association between PT quartile and massive bleeding risk. Results: PT was associated with a risk of massive intraoperative bleeding. The sensitivity analysis yielded a similar result using the minimally adjusted model (p for trend = 0.001), and minimal changes were observed using the crude and fully adjusted models (p for trend = 0.001 and 0.037, respectively). The risk of major bleeding was significantly higher in the fourth quartile (Q4) versus first quartile (Q1) group (odds ratio = 2.26, p = 0.034). A linear relationship was observed between PT and the risk of massive bleeding. Conclusions: PT was independently and linearly associated with the risk of massive bleeding. The risk of intraoperative hemorrhage was significantly higher in the higher PT (Q4) than lower PT (Q1) group. Clinical Trial Registration: The study was registered at Chinese Clinical Trial Registry (https://www.chictr.org.cn), registration number: ChiCTR2100044798.