Despite the availability of various diagnostic procedures, a tissue biopsy is still indispensable for the routine diagnosis of lung cancer. However, inaccurate diagnoses can occur, leading to inefficient cancer management. In this context, use of circulating microRNAs (miRNAs) may serve as diagnostic tools as liquid biopsies, and as biomarkers to better understand the molecular mechanisms involved in the progression of cancer. We identified miR-590-5p as a potential prognostic marker in the progression of non-small cell lung cancer (NSCLC). We were able to detect this miRNA in blood plasma samples of NSCLC patients through quantitative real-time PCR. Our data showed an ~7.5-fold downregulation of miR-590-5p in NSCLC patients compared to healthy controls, which correlated with several clinicopathological features. Further, overexpression of miR-590-5p led to decreased cell viability, proliferation, colony formation, migration, and invasion potential of lung cancer cells, whereas its knockdown showed the opposite effect. In addition, the levels of several proteins involved in the epithelial-to-mesenchymal transition negatively correlated with miR-590-5p levels in lung adenocarcinoma cells and tumors of NSCLC patients. Further, dual-luciferase reporter assays identified STAT3 as a direct target of miR-590-5p, which negatively regulated STAT3 activation and its downstream signaling molecules (eg, Cyclin D1, c-Myc, Vimentin, and β-catenin) involved in tumorigenesis. Taken together, our study suggests that miR-590-5p functions as a tumor suppressor in NSCLC through regulating the STAT3 pathway, and may serve as a useful biomarker for the diagnosis/prognosis of NSCLC, and as a potential therapeutic target for the treatment of NSCLC.