Background: Many transcription factors involved in embryonic development and reactivated in tumors are considered potential prognostic biomarkers and novel therapeutic targets in various cancers. Sine oculis homeobox homolog 1 (SIX1), a developmentally restricted transcriptional regulator, plays a critical role during tumor initiation and development. However, the prognostic value and biological function of SIX1 in non-small cell lung cancer (NSCLC) remain unclear. Methods: Bioinformatic analyses were conducted to investigate the expression of SIX1 in cancer and adjacent normal tissues of NSCLC and further explore the correlations between SIX1 expression and clinical outcomes. Western blotting and RT-PCR analysis were performed to detect of SIX1 expression level in NSCLC cell lines and normal bronchial epithelial cell. EdU, CCK-8, clonal formation assay, wound healing and transwell assay were performed to explore the effects of gain- or loss-of-function of SIX1 on cellular proliferation, migration and invasion in vitro. Gene set enrichment analysis (GSEA) was used to identify the potential signaling pathways involved in SIX1 mediated biological function and the correlation was confirmed by western blotting and RT-PCR analysis. In vivo experiment was conducted to further validate the tumor-promoting effects of SIX1. Results: Bioinformatic analysis indicated that SIX1 was markedly upregulated in NSCLC tissues of and positively correlated with poor prognosis of patients with NSCLC. Ectopic expression of SIX1 facilitated proliferation, migration, invasion, and epithelial-mesenchymal transition (EMT) of NSCLC cells. On the contrary, knocking down SIX1 exhibited the opposite effects. Mechanistic studies suggested that SIX1 activated the Notch pathway to promote the malignant biological behaviors of NSCLC, which could be reversed by inhibiting the Notch signaling with gamma-secretase inhibitor. Conclusions: SIX1 could facilitate multiple malignant biological behaviors by activating the Notch signaling pathway and function as a promising prognostic biomarker.