The melt viscosity (η) of anhydrous and hydrous peridotite was investigated using a multipronged approach combining micropenetration viscometry, conventional DSC, flash DSC and Brillouin spectroscopy. Raman spectroscopy measurements were used to verify the absence of crystallization and/or degassing during high-temperature measurements of these extremely reactive glasses and melts, ensuring that the data corresponded to the crystal-free melt viscosity. Based on the experimental data, an accurate description of the viscosity of peridotite melts over thirteen orders of magnitude (from Tg to η ≈ 10−1 Pa s) is provided in a broad range of compositions and oxidation states. Since empirical models predict viscosities that can significantly deviate from measured data to varying degrees, a new model was developed for the temperature- and H2O-dependent viscosity of peridotite melts (up to 12 mol% H2O content).