<p>Numerical modeling provides the opportunity to quantify the reaction of lakes on alterations in their environment, such as changes in climate or hydrological conditions. The one-dimensional hydrodynamic General Lake Model (GLM) is an open-source software and widely used within the limnological research community. Nevertheless, neither an interface to process the input data and run the model, nor tools for an automatic parameter calibration exist. Hence, we developed <i>glmGUI</i>, a Geographical User Interface (GUI) including a toolbox for an autocalibration, parameter sensitivity analysis, and several plot options. The tool is provided as a package for the freely available scientific code language R. The model parameters can be analyzed and calibrated for the simulation output variables water temperature and lake level.</p> <p>The <i>glmGUI</i> package is tested for two sites (Lake Ammersee, Germany, and Lake Baratz, Italy) distinguishing in size, mixing regime, hydrology of the catchment area (i.e. the number of inflows and their runoff seasonality), and climatic conditions. A robust simulation of water temperature for both lakes (Ammersee: RMSE = 1.17 °C, Baratz: RMSE = 1.30 °C) is achieved by a quick automatic calibration. The quality of a water temperature simulation can be assessed immediately by a difference plot provided by <i>glmGUI</i>, which displays the distribution of the spatial (vertical) and temporal deviations. The calibration of the lake level simulations of Lake Ammersee for multiple hydrological inputs including also unknown inflows yielded a satisfactory model fit (RMSE = 0.20 m). This shows that GLM can be also used to estimate the water balance of lakes correctly. The tools provided by <i>glmGUI</i> enable a less time-consuming and simplified parameter optimization within the calibration process. Due to this, the free availability and the implementation in a GUI, the presented R package expands the application of GLM to a broader field of lake modeling research and even beyond limnological experts.</p>