In this study, we aimed to compared the performance of [18F]FAPI-04 and [18F]fluorodeoxyglucose (FDG) positron emission tomography/computer tomography (PET/CT) in the evaluation of peritoneal carcinomatosis.71 patients with suspected peritoneal malignancies were enrolled in our study. All the participants underwent both [18F]FAPI-04 and [18F]FDG PET/CT imaging within 7 days. The detection rates, diagnostic accuracies, semiquantitative parameters of the tracers, peritoneal cancer index (PCI) scores, and tumor markers were evaluated and compared.Among the 71 patients, 40 patients were diagnosed with peritoneal carcinomatosis, and 31 were true-negative patients. The sensitivity and accuracy of [18F]FAPI-04 PET/CT were higher than those of [18F]FDG PET/CT (sensitivity: 92.50% vs. 72.50%, p=0.003; accuracy: 91.55% vs. 80.28%, p<0.001), particularly in patients with gastric cancer. The SUVmax, tumor-to-liver background ratio (TBR-L), tumor-to-descending aorta ratio (TBR-A), and PCI score were significantly higher for [18F]FAPI-04 PET/CT than [18F]FDG PET/CT (all p<0.05). In the [18F]FAPI-04 PET/CT group, the PCI score, TBR-L, TBR-A, TBR-M and SUVmax were higher in the high level group than the low level group (all p<0.05). The carbohydrate antigen 125 (CA 125) levels were strongly correlated with the PCI of both [18F]FAPI-04 and [18F]FDG PET/CT.[18F]FAPI-04 PET/CT outperformed [18F]FDG PET/CT in the evaluation of peritoneal carcinomatosis, particularly in patients with gastric cancer. Furthermore, [18F]FAPI-04 PET/CT may be used for the assessment of peritoneal carcinomatosis in patients, especially FAPI-PCI.Copyright © 2025 The Association of University Radiologists. Published by Elsevier Inc. All rights reserved.