Since masonry is one of the oldest and most traditional construction types, corresponding safety concepts are usually based on experience instead of being calibrated by structural reliability methods. For this reason, reliability analyses of masonry structures are needed to check if safety factors should be adjusted. Masonry is a non-homogenous material. Because of that, it is very important to consider the spatial variability of material properties when assessing the reliability of masonry walls. Therefore, it is useful to know if and to what extent spatial variability increases or decreases the reliability of masonry walls and the required safety factors. The influence of spatial variability depends on the length of a wall due to the capability of load redistribution. Also, it is affected by the governing failure mode, which depends on the slenderness of the wall, and can be local compression or stability failure. This paper demonstrates the effect of spatial variability on the load-bearing capacity of masonry walls in terms of mean value, scatter and design value. For this purpose, walls of varying length and slenderness were analysed with and without the consideration of spatial variability by performing Monte Carlo simulations. Based upon that, safety factors were determined which are required to meet the target reliability defined by EN 1990.