This research aimed to determine the isothermal drying kinetic parameters of grain sorghum using a thermogravimetric analyzer (TGA). The kernels were placed in the TGA under isothermal drying conditions, i.e., 40, 50, 60, 70, 80, 90, and 100 °C. Changes in the sample weight were determined from the TGA and the data were used to determine the moisture ratio and the derivative of the weight loss curves. The moisture ratio data obtained experimentally were fitted to four well-known models, namely Page, Newton, Logarithmic, and Henderson, to determine the best-fit model for the experimental data. The goodness of fit criteria was used to determine the best-fit model. An increased drying temperature from 40 °C to 100 °C accelerated the drying process and decreased the moisture ratio from 0.6091 to 0.2909, after 1 h. The Page model was the best fit for 71.4% of the drying curves, whereas the Logarithmic and Henderson models were the best fit for 28.6% of the studied cases. Increasing the drying temperature from 40 °C to 100 °C increased the effective moisture diffusivity from 0.96 × 10−8 m2/s to 1.73 × 10−8 m2/s. The drying activation energy value reached 9.4 kJ/mol under isothermal drying conditions.