Background: Immunohistochemistry (IHC) for HER2 in E-EMCA is not standard of care. We aimed to establish the correlation of HER2 transcript to IHC expression in the much more frequently tested uterine serous carcinoma (USC). We applied the threshold calculated in USC to E-EMCA and compared molecular and immune profiles among HER2+ and HER2- E-EMCA tumors, which may affect response to targeted therapy. Methods: 1462 E-EMCA tumors were analyzed using next-generation sequencing (592, NextSeq; WES, NovaSeq), WTS (NovaSeq) (Caris Life Sciences, Phoenix, AZ). PD-L1 was tested by IHC (SP142, >1%). Microsatellite instability (MSI) was tested by FA, IHC and NGS. TMB was measured by totaling somatic mutations per tumor (TMB-H: >10 mutations/MB). LOH cut-off was > 16%. HER2+ cut-off by WTS was determined by Receiver Operator Characteristic (ROC) analysis in USC tumors by comparing to HER2 IHC/CISH results using 2018 Breast Cancer ASCO/CAP Guidelines. Immune cell infiltrates were calculated by Quantiseq. Real world overall survival (OS) was extracted from insurance claims data and calculated using Kaplan-Meier survival curves for molecularly defined cohorts. Significance was determined using chi-square and Wilcoxon rank sum test and adjusted for multiple comparisons (q-value < 0.05), p < 0.05 but q > 0.05 was considered a trend. Results: We determined a cut-off of > 62.99 TPM for HER2+ with a sensitivity of 81.5%, specificity of 87.6% and AUC of 0.92 in Uterine Serous. When this cut-off is applied to E-EMCA, 76 of 1462 (5.2%) E-EMCA tumors were HER2+. HER2+ tumors had fewer mutations (mt) in PI3KR1, PTEN and CTNNB1 but higher mts in TP53 and more frequent LOH (q < 0.05). HER2+ tumors had a trend towards decreased MSI-H status (22.4% vs 39.1%; p = 0.003, q = 0.058) and TMB-H (25.4% vs 41.5%; p = 0.007, q = 0.084). MSS HER2+ E-EMCA had a similar mutational profile compared to all HER2+ tumors; MSI-H HER2+ E-EMCA had a trend towards higher DDR pathway gene mts compared to MSI-H HER2- EMCA tumors. HER2+ tumors had increased Dendritic cell (3.84% vs 2.97%) but decreased Neutrophil (2.66% vs 5.20%) and T-reg (1.38% vs 2.07%) infiltration (q < 0.01). HER2+ tumors had higher immune checkpoint gene expression of CD80, HAVCR2 and PDCD1LG2 (q < 0.01), and increased T-cell inflamed and MAPK activation score (q < 0.01). MSS HER2+ E-EMCA tumors had a similar immune profile when compared to all HER2+ tumors; MSI-H HER2+ E-EMCA tumors had increased Treg infiltration and MAPK activation score. Median OS was significantly worse for HER2+ pts compared to HER2- (64.3 vs. 23.6 months, HR: 1.93(1.32-2.80), p < 0.001). Conclusions: Using a WTS cutoff from USC, we found 5% of E-EMCA are HER2+ and showed distinct molecular and immune profile compared to HER2- tumors. HER2+ confers a worse OS compared to HER2- tumors. Furthermore, HER2+ tumors demonstrate an immune hot phenotype suggesting that immunotherapy may be a potential therapeutic option.