The beneficial effect of plants in treating diabetes is not only well-known in traditional medicine but also confirmed in numerous scientific studies. The basic platform for testing the potential antidiabetic activity of traditionally known plants and their bioactive compounds is a set of in vitro, in vivo experiments, clinical trials, and molecular docking studies. Basic assays usually measure enzyme inhibitory activity (α-amylase and α-glucosidase) and other aspects related to diabetes mellitus disease. Recently, the use of plant-derived compounds has proven useful in treating diabetes and reducing complications resulting from high blood sugar levels. The main goal is to establish an action mechanism of plant extracts or active compounds to find new antidiabetic drugs with less toxicological properties. This work aims to collect data and discuss the newest results in the area of plant extracts, compounds, and antidiabetic effects using in vitro, in vivo, and in silico models. The data covered in this review include plant extracts, polyphenols, terpenoids, saponins, phytosterols, and other bioactive compounds, with some of the investigated plants being less known. Isolation of new compounds might be a plentiful source for treatment and prevention of diabetes mellitus. Clinical trials with adequate monitoring give the best results of plants' product efficacy and safety. Many studies have confirmed the importance of patent and use of medicinal herbs in the treatment of diabetes.