Gastric cancer is a commonly diagnosed, often fatal malignancy and requires novel anticancer therapies and preventative approaches. This study described the involvement of MAFG-AS1, a lncRNA with important functions in cancer biology, in gastric adenocarcinoma (GA). Thirty-six male and forty-two female GA patients with an average age of 51.9 ± 5.7 years in the range of 35 to 68 years were enrolled. Paired gastric cancer (GC) and non-tumor tissues were collected from each patient. MAFG-AS1 expression was determined. RNA interaction prediction, dual luciferase reporter assay, RT-qPCR assay, Western blot, and CCK-8 assay were conducted. The results indicated that MAFG-AS1 was highly expressed in GA and closely correlated with poor survival. MAFG-AS1 interacted with miR-505, but MAFG-AS1 and miR-505 overexpression showed no significant effects on each other's expression. In addition, MAFG-AS1 increased the expression of PLK1, a miR-505 target. MAFG-AS1 and PLK1 overexpression increased GC cell proliferation rate. MiR-505 overexpression reduced the effects of MAFG-AS1 and PLK1 overexpression on cell proliferation. Therefore, MAFG-AS1 might upregulate PLK1 by sponging miR-505 to promote GA cell proliferation.