The boundary layer on the endwall of a turbine blade cascade is subject to cross-stream pressure gradients in the blade passage, which generate a cross-stream velocity component to make it three-dimensional. This distorts the turbulence relative to a two-dimensional boundary layer, and impacts the endwall heat transfer. This study presents measurements of the three-dimensional boundary layer in a turbine cascade obtained with a laser Doppler velocimeter. In addition, two types of RANS models are compared to the measurements: the SST k-ω model using the isotropic eddy viscosity assumption, and a Reynolds stress model which allows for anisotropy of the Reynolds stress. Neither model fully captures the complexity of the three-dimensional boundary layer in a turbine blade passage, particularly for turbulence associated with the cross-stream flow and for the highly accelerated 3D boundary layer at the passage exit.