This paper aims to investigate the reliability of strip footing in the presence of spatially variable undrained shear strength that linearly increases with depth. A non-stationary random field is used to model the spatially varying undrained shear strength. A strip footing example is presented to investigate the effect of spatially variable undrained shear strength on the performance of strip footing. The results indicate that the mean bearing capacity for spatially variable undrained shear strength is smaller than that obtained from a deterministic analysis. Both the mean and standard deviation of bearing capacity increase with increasing autocorrelation length. Ignoring the trend of undrained shear strength linearly increasing with depth will significantly overestimate the probability of failure of the strip footing. A factor of safety significantly below 3.0 may be used for designing strip footings if the trend of undrained shear strength linearly increasing with depth is considered properly.