Background: Metastatic breast cancer (MBC) is a heterogeneous disease associated with known somatic mutations of variable biological value in different subtypes. Furthermore, the clinical evolution of the disease demonstrates clonal evolution resulting in disease resistance more accurately detected using blood-based sequencing. Few studies have explored differences in genomic features of tumors across populations. Here, we performed circulating tumor DNA (ctDNA) sequencing to compare the genomic landscape of patients with hormone-receptor positive MBC at time of first recurrence or de-novo metastatic diagnosis in the United States (US) and China. Methods: Twenty-three US patients from Northwestern University and 65 Chinese patients from Peking University had ctDNA sequencing from plasma performed using the harmonized CLIA-certified, 152-gene PredicineCARE assay in laboratories in the US and China, respectively. The data analysis was conducted in China. Institutional Review Boards at each site approved the study. Fisher’s exact test was performed to compare mutational frequencies across populations. Results: Median age of patients at MBC diagnosis was 51 in the US cohort and 55 in the Chinese cohort. 87% of US patients and 82% of Chinese patients had received prior therapy for primary breast cancer, including endocrine therapy. Mutations were detected in 17 of 23 (74%) US patients and 59 of 65 (91%) Chinese patients. CNAs were observed in 57% of US patients and 58% of Chinese patients. The most common mutations detected in US patients were TP53 (26%), PIK3CA (22%), AKT1 (22%), CDH1 (17%), PTEN (13%), and ESR1 (9%) vs. PIK3CA (46%), TP53 (35%), ESR1 (12%), and BRCA2 (11%) in Chinese patients. Frequency of AKT1 and CDH1 mutations were significantly higher in the US population (P < 0.05), while PIK3CA mutations were higher in the Chinese population (P < 0.05). CNA gains in CCND3 and CDK4 were significantly higher in the US cohort, and FGFR1 was significantly more common in the Chinese cohort (all P < 0.05). Conclusions: To our knowledge, this is a first cross-regional comparison study in HR+ MBC patients in the US and China using a harmonized cfDNA NGS platform. At a population level, there were notable differences observed in somatic variants in two cohorts. Future sequencing efforts and clinical trials should include patients of diverse ethnic backgrounds to explore the impact of differences in genomic landscape on probability of benefit from treatments. A larger validation cohort is required to confirm these findings.