Specimens of Nb3Sn tape have been irradiated at ∼380 K with reactor-spectrum neutrons to fluences of 1.3×1019 n/cm2 (E>1 MeV) and the internal friction and dynamic Young’s modulus measured from 6 to 300 K. The softening of the lattice with decreasing temperature is seen to decrease from 40% of the room-temperature value at 10 K for unirradiated Nb3Sn to 94% at a fluence of 1.3×1019 n/cm2. The internal friction δ due to tetragonal domain-wall motion of the transformed material decreased with increasing fluence, being completely suppressed after ∼4×1018 n/cm2. The transformation temperature Tm decreased from ∼49 K for the virgin specimen to 22 K for a fluence of 3.8×1018 n/cm2 with no evidence of any tetragonal transformation at all for higher fluences. The lattice-dynamical and electronic changes responsible for these effects are not known at this time, but are seen to be correlated with the neutron-induced disorder.