Knowledge of the strength and elastic modulus of a reservoir rock is important for the optimisation of the geothermal resource, but is rarely considered during exploration and rock characterisation studies. This is especially true over intervals with high fracture densities, where the rock mass strength and stiffness will be significantly lower than the intact rock strength and stiffness. Here we apply rock engineering techniques for assessing rock mass strength and stiffness to the reservoir rock for the geothermal systems in the Upper Rhine Graben, such as those at Soultz-sous-Forêts and Rittershoffen (both France). We couple uniaxial and triaxial deformation experiments performed on intact rock with Geological Strength Index assessments—using the wealth of information from core and borehole analyses—to derive the generalised Hoek–Brown failure criterion and provide rock mass strength and elastic modulus estimates for the granite reservoir and overlying sandstone rocks at Soultz-sous-Forêts (from a depth of 1012 to 2200 m). The fracture densities in this reservoir granite range up to ~ 30 fractures/m. The average intact uniaxial compressive strength (UCS) and elastic modulus of the granite are 140 MPa and 40 GPa, respectively, while for the dry sandstone they range from 50-250 MPa and 10-40 GPa, respectively, depending on the unit. Intact strength increases with confining pressure (i.e. depth). For the granite, the intact UCS is quite homogeneous. The modelled strength of the intact granite is 360 MPa at a depth of 1400 m and increases to 455 MPa at a depth of 2200 m (using our estimate for the empirical mi term of 30, determined using triaxial and tensile strength measurements on the intact granite). For the sandstone, intact UCS is highly variable, and while it also increases with confining pressure, this does not translate to a direct relationship with depth because the UCS is unit-dependent, and the unit strengths decrease with depth. The depth-correct intact rock strength of the dry sandstone varies from 150-350 MPa (using an mi of 19). Strength of the rock masses vary in accordance with the intact strength, the fracture density and the extent and nature of the fracture infill, reaching lows of ~ 30 MPa (in, for example, the densely fractured sandstone in exploration well EPS-1 (at Soultz-sous-Forêts) at a depth of ~ 1115 m) and highs of above 400 MPa (in, for example, the largely unfractured granite at a depth of ~ 1940–2040 m). Variations in rock mass elastic modulus are qualitatively similar (values vary from 1 to 2 GPa up to the elastic modulus of the intact rock, 40 GPa). Our study highlights that macrofractures and joints reduce rock mass strength and elastic stiffness and should be considered when assessing the rock mass for well stability and rock mass deformation due to stress and pressure redistribution in the reservoir. We present this case study to demonstrate how a simple and cost-effective engineering method can be used to provide an indication of the in situ strength and elastic modulus of reservoir rock masses, important for a wide range of modelling and stimulation strategies. We recommend that the effect of macrofractures on rock mass strength and stiffness be validated for incorporation into geomechanical characterisation for geothermal reservoirs worldwide.