Exosomal microRNAs (miRNAs) have great potentials as a novel biomarker to predict lung cancer. We applied a miRNA microarray to identify aberrantly expressed serum exosomal miRNAs as candidate biomarkers for patients with lung adenocarcinoma (LUAD). Compared with the normal control, 31 exosomal miRNAs were found to be upregulated and 29 exosomal miRNAs were downregulated in the serum of LUAD respectively. Then, 10 dysregulated exosomal miRNAs expression levels in serum were further validated via qRT‐polymerase chain reaction. Notably, exosomal miR‐7977 was highest expressed and miR‐98‐3p was lowest expressed in the patients with LUAD, and exosomal miR‐7977 showed significant correlation with the N stage and TNM stage with patients with LUAD (P < .05). Receiver operating characteristic curve showed that the abundant level of exosomal miR‐7977 may predict LUAD with an area of under the curve (AUC) of 0.787. In comparison with exosomal miR‐7977, exosomal miR‐98‐3p had a smaller area (0.719). The combination of exosomal miR‐7977 and miR‐98‐3p improved the AUC to 0.816. Furthermore, in vitro experiments revealed that inhibition of miR‐7977 enhanced the proliferation, invasion, and inhibited apoptosis in A549 cells, the opposite results were performed by miR‐7977 mimics. In conclusion, exosomal miR‐7977 was identified as a novel biomarker for patients with LUAD and may play as a tumor suppressor in lung cancer. We were the first time to provides evidence that serum exosomes‐derived miR‐7977 is a novel promising diagnostic biomarker for lung adenocarcinoma and may play a tumor suppressor in lung cancer.