Gastric gland mucin consists of core protein MUC6 with residues heavily glycosylated by unique O-glycans carrying α1,4-linked N-acetylglucosamine (αGlcNAc). αGlcNAc-glycosylated MUC6 protein is seen in normal gastric and duodenal glands. Decreased αGlcNAc glycosylation on MUC6-positive tumor cells is often observed in premalignant lesions of the stomach, pancreas, and bile duct, and decreased MUC6 expression is seen in invasive cancer of these organs. Lung cancer (LC) is the most common cause of cancer death worldwide. Recently, the adenocarcinoma subtype has become the most common histological subtype of LC, and one of its invasive forms is invasive mucinous adenocarcinoma (IMA). Currently, prognostic markers of LC IMA are unknown. Here, we analyzed MUC5AC, MUC6, and αGlcNAc expression in 54 IMA LC cases. MUC5AC was positively expressed in 50 (93%), MUC6 in 38 (70%), and αGlcNAc in 19 (35%). Each expression level was scored from 0 to 3. The αGlcNAc expression score was significantly decreased relative to MUC6 (P < 0.001). Interestingly, disease-free survival was significantly higher in MUC6-positive than MUC6-negative cases based on the log-rank test (P = 0.021). For in vitro analysis, we ectopically expressed MUC6 in A549 cells, derived from LC and harboring a KRAS mutation. MUC6-expressing A549 cells showed significantly lower proliferation, motility, and invasiveness than control cells. Finally, F-actin staining in MUC6-expressing cells revealed a decrease or loss of filopodia associated with decreased levels of FSCN transcripts, which encodes an actin-bundling protein fascin1 necessary for cell migration. We conclude that MUC6 expression is a preferable prognostic biomarker in IMA LC.