The magnetism in SrFe2As2 can be suppressed by electron doping through a small substitution of Fe by Co or Ni, giving way to superconductivity. We demonstrate that a massive substitution of Fe by isovalent ruthenium similarly suppresses the magnetic ordering in SrFe2-xRuxAs2 and leads to bulk superconductivity for 0.6 < x < 0.8. Magnetization, electrical resistivity, and specific-heat data show T-c up to approximate to 20 K. Detailed structural investigations reveal a strong decrease in the lattice parameter ratio c/a with increasing x. Density functional theory band-structure calculations are in line with the observation that the magnetic order in SrFe2-xRuxAs2 is only destabilized for large x.