Background: Appendiceal adenocarcinoma (AA) is a rare malignancy with distinct clinical and molecular characteristics compared to colorectal cancer (CRC). Currently there is little knowledge regarding how tumor cells from AA interact with the tumor microenvironment (TME). In this study, we performed single cell RNA sequencing (scRNAseq) on metastatic AA tumors taken from three patients undergoing curative intent surgical resection. Methods: After obtaining informed consent, tumor tissue from three patients with AA and one patient with CRC undergoing cytoreductive surgery at the MD Anderson Cancer Center (MDACC) was harvested and dissociated into single cell suspension for scRNAseq; libraries were prepared with ‘5 sequencing (10X Genomics). Bulk RNA sequencing from 19 additional patients with AA was available as a validation cohort. Results: scRNAseq analysis of a cohort of 3 patients with AA, 1 with CRC, and 12 healthy control samples identified 39,451 evaluable single cells after filtering and doublet removal. Separating cells into immune, stromal, and epithelial compartments identified that epithelial cells are rare in AA tumors. On average, AA constituted 62% stroma, 36% immune and only 2% epithelial cells, in contrast to healthy appendix (46% immune, 29% epithelial, 25% stroma). Sub-classification of immune cells revealed enrichment of myeloid cells in AA tumors (67% myeloid, 27% T, 5% B and 1% plasma B cells). This is in contrast to healthy appendix tissue (44% T, 26% plasma B, 22% B, 6% myeloid and 2% mast cells). In CRC, the majority of immune cells from the CRC sample were T cells (69%). These single cell data were used to improve deconvolution of bulk RNAseq, allowing for evaluation of TME contribution to transcription using bulk RNAseq data. Analysis of tumors from 19 patients with AA confirmed myeloid predominance in AA tumors. We performed pseudobulk differential expression analysis between compartments comparing CRC and AA which identified significant differences between compartments. Gene set enrichment analysis enabled identification of organized compartment specific changes. Conclusions: Findings from this study provide new insights into AA's biological distinctions from CRC and represent a crucial step toward better understanding this rare malignancy. Myeloid cells need further investigation in AA to develop specialized management approaches and improve patient outcomes.