Extremely acid saline lakes and groundwaters on Earth may be good models for possible Martian environments. Terrestrial sedimentary acid systems include both Permian deposits in the U.S. midcontinent and modern environments in southern Australia. These terrestrial acid systems are characterized by: (1) saline H2SO4-rich lake and ground waters with pH between 4 and -1; (2) bedded and displacive evaporite minerals hosted by red siliciclastic sediments; (3) common sulfate minerals, such as gypsum and anhydrite, and less abundant "acid" sulfate minerals, such as alunite and jarosite; (4) possible acidophilic microorganisms; and (5) lack of carbonate minerals. These acid saline lakes are surrounded by mudflats, sand flats, ephemeral streams, sand dunes, and/or distal alluvial fans. Variations in these terrestrial acid saline environments include range of pHs from extremely low (+1 to -1) in some Permian deposits to moderately low (+4 to +2) in some modern Australian environments. Also, minor variations exist in the evaporite mineralogy and water geochemistry among and between ancient and modern acid deposits. We have made preliminary comparisons between these environments and data concerning the Martian surface. Many of the same sedimentary features and landforms seen on Mars are also seen on a smaller scale in and near terrestrial acid saline environments. Chemical and mineralogical data from Mars closely resemble that of terrestrial acid deposits. Both Martian soil and terrestrial acid sediments are rich in quartz and iron oxides, have evaporite minerals such as halite, gypsum, and anhydrite, and contain some rare "acid" sulfate minerals such as jarosite. Carbonates, common on Earth, are lacking in both terrestrial acid settings and Martian sediments. These observations suggest that terrestrial acid saline environments should be better studied as possible analogs for Martian environments.