In this paper, a two-step optimization methodology for the design and off-design optimization of low-temperature (110-150 degrees C) geothermal organic Rankine cycles (ORCs) is proposed. For the investigated conditions which are based on the Belgian situation we have found that the optimal ORC design is obtained for design parameter values for the environment temperature and for the electricity price which are both higher than the respective yearly-averaged values. However, the net present value is negative (-12.62 MEUR) which indicates that the low-temperature (130 degrees C) geothermal electric power plant is not economically attractive for the investigated case. Nevertheless, and demonstrated by the results of a detailed sensitivity analysis, a low temperature geothermal power plant might be economically feasible for geological sites with a higher brine temperature or in a country with a more favorable economic situation; e.g., with higher electricity prices (similar to 70 EUR/MWh). The novelty of our paper is the development of a thermoeconomic design optimization strategy for low-temperature geothermal ORCs, accounting for the off-design behavior already in the design stage. The generic methodology is valid for low-temperature geothermal ORCs (with MW scale power output) and includes detailed thermodynamic and geometric component models, is based on hourly data rather than monthly-averaged data and accounts for economics.