This study was aimed to investigate chemical composition, biological activity, and food protecting ability of Quercus infectoria gall (QIG).The total content of phenolics, flavonoids, and hydrolyzable tannins was evaluated. Ion and fatty acid compositions of QIG were identified by ion chromatography and gas chromatography flow ionization detector (GC‐FID), respectively. Ellagic acid (28,156.85 mg/kg) and catechin (716.21 mg/kg) were predominant. The antioxidant activity of QIG was determined as 2.29, 1.65, 1.52, and 1.98 mmol Trolox equivalent antioxidant capacity (TEAC)/g sample for DPPH, ABTS, FRAP, and CUPRAC analyses, respectively. QIG extract exhibited dose‐dependent inhibition on α‐glucosidase enzyme with an IC50 value of 0.002 mg/ml. QIG displayed antimicrobial activity against selected reference and isolated mixed strains, most notably on Escherichia coli. The addition of QIG extract as a natural preservative to pasteurized cow’s milk resulted in lower total bacterial count (TBC), total yeast‐mold count (TYMC), and higher pH compared to control milk. Practical applications Quercus infectoria gall does not have a common use in the food and pharmaceutical industry. Exploration of the the biological value of Q. infectoria gall or its extracts could pave the way for the development of added value natural food preservatives and functional compounds. The chemical composition of Q. infectoria gall should be characterized and its biological functionality should be demonstrated beforehand. The results indicated that this plant could be considered as natural antidiabetic, antioxidant, and protective agents instead of synthetic materials and ingredients for food, cosmetic, and pharmaceutical applications.