Period2 (Per2) is a key circadian clock gene, and its deregulation contributes to tumour development, including breast cancer. However, the biological function and clinicopathological significance of Per2 in non-small cell lung cancer (NSCLC) remain unclear. The present study aimed to explore the role of Per2 and its relative clinical significance in NSCLC. To analyse Per2 expression in NSCLC specimens, reverse transcription-quantitative polymerase chain reaction was performed, and the results indicated that Per2 expression was markedly downregulated in 83.87% (26/31) of NSCLC samples compared with their adjacent matched tissues. Increased Per2 expression was associated with increased differentiation (P<0.01) and reduced lymph node metastasis (P<0.0001). Functional studies identified that enhancing Per2 expression in A549 cells by lentivirus transduction not only significantly suppressed cell growth, migration and invasion (P<0.05) but also inhibited NSCLC growth and metastasis in vivo. Animal studies and histopathological analysis identified that Per2 expression in A549 cells not only markedly increased expression of tumour anti-oncogenes Bax, P53 and P21 but also inhibited expression of pro-oncogenes vascular endothelial growth factor, CD44 and c-Myc. These results indicate that the loss of Per2 is one of the factors underlying tumourigenesis in NSCLC, and it may function as a novel molecular target for NSCLC.