Effects of nonmagnetic scandium octahedral substitution in yttrium iron garnet were studied in a series of polycrystalline garnets having the unit formula Y3ScxFe2−xFe3O12wherexwas varied from 0 to 1.7. Measurements of the lattice parameters, spontaneous magnetization, Curie temperature, and the initial permeability as a function of frequency and temperature are reported. The lattice parameter varied linearly from 12.376±0.003 Å forx=0(YIG) to 12.495±0.005 Å forx=1.5 Sc3+. Single‐phase garnet was not obtained forx>1.5. The apparent spontaneous magnetization atT=0°K and the Curie temperature dependence on substitution are qualitatively explained using Gilleo's statistical theory of incomplete super‐exchange interactions. The initial permeability at room temperature is strongly affected by the addition of scandium, increasing from 120 (YIG) to a maximum value of 1934 (x=0.9 Sc3+) and then to unity for largerxvalues. The value of 1934 is the largest room‐temperature initial permeability reported in garnets to date; the corresponding Curie temperature is 320°K. The frequency spectra of the initial permeability forx≤0.7 Sc3+display the usual two characteristic loss peaks identified with domain wall motion and domain rotation mechanisms. The high‐frequency (domain rotation) peak appears only as a shoulder on the broad wall motion resonance. The wall motion peak does not exhibit electronic diffusion effects. In the absence of experimental anisotropy data on Sc3+substituted garnets, the temperature dependence of the initial permeability was used to deduceK1(T) by assuming that the rotational permeability contributes ∼20% to the total measured permeability. The rotational resonance frequencies calculated using the deduced anisotropy constants at room temperature agree quite well with experimental data. The derivedK1(T) seem to be in good agreement with the available single‐crystal experimental data.