Background: Appendiceal adenocarcinoma (AA) is a rare and heterogenous cancer with marked differences in clinical course between high- and low-grade tumors. Unlike colorectal cancer (CRC) and other gastrointestinal malignancies, AA virtually never has hematogenous metastases, rather, metastasis is limited to the peritoneum. Here we present a retrospective, single institution study of AA to identify the prevalence of detectable ctDNA, evaluate the clinical predictive value of positive ctDNA, and assess what clinical, pathologic, or molecular features predict positive ctDNA. Methods: 160 blood samples from 147 patients with AA metastatic to the peritoneum were profiled with a CLIA approved 73 gene mutational panel as part of routine clinical practice. Paired tumor sequencing was available for 73 patients. Survival was measured starting from day ctDNA was drawn. Mutations that most likely represented clonal hematopoiesis were removed from analysis. Results: Out of 160 ctDNA samples, 120 were taken in the setting of radiographically apparent metastatic disease. Of these, 46 (38.3%) had any detectable mutation. 40 ctDNA tests were performed when patients had no radiographic evidence of disease (NED); 15 (37.5%) of which had any detectable mutation. High-grade tumors were more likely to have detectable ctDNA with detection rates of 10/46 (21.7%), 18/46 (39.1%), and 33/68 (48.5%) for well, moderately, and poorly-differentiated tumors, respectively. Restricting analysis to the 73 patients with paired tissue and blood samples and 73 genes sequenced in both, of 81 mutations detected in tumor only, 21 were detected in blood (sensitivity of 26%). Sensitivity was highest for mutation in TP53 (53.8%) suggesting these tumors may have a greater propensity to shed DNA into circulation. Overall, the sensitivity of ctDNA detection in metastatic AA was markedly less than what was observed in a cohort of 274 metastatic CRC patients from the same institution (288/581 = 49.6%). For AA patients with detectable ctDNA, variant allele frequency (VAF) in AA was significantly lower compared to CRC (median VAF 0.04% vs. 6%, p = <0.0001). Detectable ctDNA was associated with shorter overall survival (46.2 mo for positive ctDNA vs. not-yet-reached for negative ctDNA, HR = 2.5, p = 0.016) and shorter disease free survival (DFS) (60 mo vs. not-yet-reached, HR = 3.4, p = 0.05). As expected, patients with radiographic evidence of disease did worse than patients with NED (HR = 2.1. p = 0.08), but of note this hazard ratio was less than that for positive ctDNA. Conclusions: In patients with AA, the presence of detectable ctDNA is associated with shorter overall and disease-free survival. Sensitivity of ctDNA detection in metastatic AA is overall markedly lower than CRC, with detection more likely in high-grade tumors and higher sensitivity in tumors containing TP53 mutation.