In situ high-temperature (1165–1248 K) scanning-tunneling microscopy was used to measure temporal fluctuations about the anisotropic equilibrium shape of two-dimensional TiN(111) adatom and vacancy islands on atomically smooth TiN(111) terraces. The equilibrium island shape was found to be a truncated hexagon bounded by alternating 〈110〉 steps, which form [100] and [110] nanofacets with the terrace. Relative step energies β as a function of step orientation φ were obtained from the inverse Legendre transformation of the equilibrium island shape to within an orientation-independent scale factor λ, the equilibrium chemical potential of the island per unit TiN area. We find that for alternating S1 and S2 〈110〉 steps, the ratio β1/β2=0.72±0.02. The parameter λ and, hence, absolute orientation-dependent values of β(φ) and step stiffnesses β̃(φ) were extracted from quantitative shape fluctuation data using an exact theoretical approach valid for anisotropic islands. For the two 〈110〉 steps, we obtain β1=0.23±0.05 and β̃1=1.9±1.1eV/Å with β2=0.33±0.07 and β̃2=0.08±0.02eV/Å over the observed temperature range. Due to the correspondingly high kink energies, TiN(111) step energies exhibit only a very weak temperature dependence between 0 K and the maximum measurement temperature 1248 K.