The Department of Energy (DOE), Geothermal Technologies Office (GTO) launched a multi-phase funding program to advance technologies for extraction of lithium from geothermal brines. This initiative, known as American-Made Geothermal Lithium Extraction Prize (GLEP), had objectives of advancing technology for direct lithium extraction (DLE) from geothermal brines and make it as cost competitive as the conventional lithium extraction methods. To support these GLEP projects, Idaho National Laboratory (INL) formulated a Synthetic Li Prize Brine (SLPB) and provided it to all Phase 3 finalists to test with their technologies. The SLPB was used as a baseline lithium extraction feed brine for testing the efficacy of direct lithium extraction (DLE) technology developed by finalists. Included here are details on the synthesis of the SLPB.