A turbine rotor blade including an airfoil that extends from a platform. The platform may include a first portion of a nominal platform contour substantially in accordance with Cartesian coordinate values of X′, Y′, and Z′ as set forth in Table II. The Cartesian coordinate values of X′, Y′, and Z′ are non-dimensional values from 0% to 100% convertible to dimensional distances by multiplying the Cartesian coordinate values of X′, Y′, and Z′ by a height of the airfoil defined along a Z′ axis. The X′ and Y′ values of the first portion are coordinate values that, when connected by smooth continuing arcs, define contour lines of the first portion of the nominal airfoil profile at each Z′ coordinate value. The contour lines may be joined smoothly with one another to form the first portion.