This study provides geotechnical characteristics of recent lahar deposits on which the city of Arequipa (South Peru) is built. Geological and sedimentological observations point out the existence of three types of lahar deposits in the Arequipa region: fine hyperconcentrated-flow deposits, coarse hyperconcentrated-flow deposits, and debris-flow deposits. The mineral components identified in the three types of lahars show that they are linked to the outcropping volcanic rocks around Arequipa city. Physical measurements (dry density, grain-size distribution, specific surface of the grains based on methylene blue tests) and mechanical tests (in situ dynamic cone penetration soundings, oedometric and Casagrande shear-box tests) were performed on the three main categories of soils. Our results highlight that hyperconcentrated-flow deposits are fine sand- and silt-rich deposits that lack clay particles. Their dry density is low (ρd = 1.25 g/cm3) and their friction angle is high (ϕ = 38°) which contribute to the peculiar dynamics of lahar flows and to their high erosive power. The low apparent density provides a better capacity for the debulking process, whereas the high friction angle takes part in the erosion process. Finally, the geotechnical properties observed here suggest that the high contents in silica pyroclastic particles and the lack of clay or fine particles control the rheological behavior of lahar deposits. We can also consider that the rheological behavior of lahars through time is complex and that existing older lahars can be remobilized by heavy rain or future stream flows and lahars.