β-glucans are a polymeric dietary fiber characterized by β-(1,3) and β-(1,4) glycosidic bonds between glucose monomers. They are often used as thickeners, stabilizers, and fat substitutes in foods. The functional and technological quality of β-glucans is attributed to their origin/source, molecular weight, and structural properties. In particular, physical treatments such as drying, cooking, freezing, and refrigeration influence their molecular, morphological, and rheological characteristics. In addition to their useful technical qualities, β-glucans are recognized for their numerous beneficial impacts on human health. For this reason, the European Food Safety Authority (EFSA) has provided a positive opinion on health claims such as cholesterol lowering and hypoglycemic properties relating to oats and barley β-glucans. This paper provides insight into the properties of β-glucans and different treatments affecting their characteristics and then reviews the latest research on β-glucans as a functional ingredient for people with type 2 diabetes mellitus (T2DM).