Background and Aims Pituitary tumor transforming gene 1 (PTTG1) is an oncogene highly expressed in various tumors and can be used as a biomarker for cancer invasion and metastasis. However, the expression level of PTTG1 in breast cancer and its relationship with the patient's prognosis is unclear. This study was conducted to investigate the relationship between the expression of PTTG1 in breast cancer and the prognosis of patients and its effect on immune cell infiltration, and preliminarily explore the possible mechanism of PTTG1 in the occurrence and development of breast cancer.Methods The expression of PTTG1 in breast cancer tissues and its prognostic value were analyzed using Oncomine 4.5 database and Kaplan-Meier Plotter database. The co-expressed genes of PTTG1 screened by Coexpedia and the enriched pathways by co-expressed genes were analyzed through GO and KEGG databases. TIMER database was used to analyze the relationship between PTTG1 gene expression level and immune cell infiltration in breast cancer. MultiMiR method of R language bundle was adopted to predict the microRNAs interacting with PTTG1 and its co-expressed genes, and Cytoscape was used for network visualization.Results PTTG1 expression was significantly increased in breast cancer tissues, and the prognosis of patients with high PTTG1 expression was significantly worse than that of patients with low PTTG1 expression (P<0.001). In breast cancer, the GO function of PTTG1 gene and its co-expressed gene set was mainly enriched in nuclear division, organelle separation, and chromosome separation. In contrast, the KEGG pathway was enriched in cell cycle, meiosis, human T lymphocytic leukemia virus type I (HTLV-1) infection and p53 signal transduction pathway. The expression level of PTTG1 was significantly positively correlated with the infiltration level of CD4+ Th1 cells (r=0.490, P=3.52e-61), CD4+ Th2 cells (r=0.765, P=3.7e-192), macrophages (r=0.308, P=2.8e-23), B cells (r=0.228, P=3.69e-13) and neutrophils (r=0.121, P=1.27e-04), but was significantly negatively correlated with the infiltration level of CD8+ T cells (r=-0.198, P=3.16e-10). The multiMiR language data package analysis showed that there were 17 microRNAs co-targeting PTTG1 and its co-expressed genes.Conclusion PTTG1 is highly expressed in breast cancer tissue and is associated with poor prognosis in patients. The expression level of PTTG1 in breast cancer is closely related to immune infiltration. High expression of PTTG1 may enhance tumor proliferation and invasion ability by regulating cell cycle and p53 signaling pathway, thereby leading to poor prognosis of breast cancer. PTTG1 may play an oncogene role in breast cancer, suggesting that PTTG1 can be used as a potential diagnostic and prognostic marker for breast cancer.