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Background: Emerging evidence suggests that circular RNAs (circRNAs) are vital regulators in a range of cancers. "miRNA sponge" is the most reported role played by circRNAs in many tumors. The insulin-like growth factor (IGF) 1 pathway plays a key role in the development and progression of many cancers, including colorectal cancer (CRC). The aim of the study is to establish the potential clinical value and driving molecular mechanisms of circRNAs in CRC. Materials and Methods: Real-time quantitative RT-PCR (qRT-PCR) was performed to measure the circRUNX1 expression in 52 tissue samples from CRC patients. We verified the tumor promotor role of circRUNX1 in cell-based in vitro and in vivo assays. Human growth factor array was used to identify circRUNX1-regulated signaling pathways. We then used a double luciferase reporter assay and RNA fluorescence in situ hybridization to identify the downstream miR-145-5p of circRUNX1. Furthermore, we performed Western blotting and biological function assays to demonstrate if the circRUNX1/miR-145-5p/IGF1 axis is responsible for the proliferation of CRC cells and promotes CRC development. Results: By performing qRT-PCR from CRC tissues and paired adjacent normal mucosa tissues, we identified that circRUNX1 expression was significantly upregulated in CRC tissues and positively related with lymph node metastasis, distant metastasis and advanced tumor-node-metastasis tumor stage in patients. Functionally, circRUNX1 knockdown inhibited cell proliferation and migration and promoted apoptosis, whereas its overexpression exerted opposite effects. In vivo, circRUNX1 promoted tumor growth and metastasis. Mechanically, circRUNX1 shared miRNA response elements with IGF1. circRUNX1 competitively bound to miR-145-5p and prevented miR-145-5p from decreasing the expression of IGF1, which facilitated tumor growth. Conclusion: Our studies verified that circRUNX1 functions as a tumor promotor in CRC cells by targeting the miR-145-5p/IGF1 signaling pathway and may have potential use as a prognostic indicator and therapeutic target in CRC patients. [ABSTRACT FROM AUTHOR]