Daily, more and more people consume snack bars that may have an impact on blood glucose levels. The aim of the present study was to compare the acute effects of a common snack and a plant-based snack bar (PB) that was developed at the University of Thessaly as a functional diabetic snack on blood glucose and insulin in patients with type 2 diabetes mellitus (T2DM). Adults with T2DM (n = 10) treated with oral medications were studied in a randomized, crossover clinical trial. On each trial day, postprandial glucose and insulin levels were measured at 30, 60, 90, and 120 min, and a morning snack containing 25 g of carbohydrate was consumed. The procedure was carried out on 2 days, with one of the test meals being consumed on each day. Consumption of a PB snack bar resulted in a smaller and steeper increase in postprandial glucose and insulin levels compared with the usual snack, and there were significant differences 60 and 90 min after consumption of the two tested snacks. The PB snack bar is rich in protein, fiber, vitamins, and minerals and can therefore be suggested as a nutritious and convenient snack for patients with T2DM. In addition, the extract of the snack bar was tested for its bioactivity in human cell cultures.