Abstract Studies of postseismic creep often infer a strong reduction in upper‐mantle viscosity after large earthquakes, yet the microphysical mechanism responsible for this behavior remains subject of debate. Here, we present new gas‐medium apparatus experiments at 1100–1200°C on pre‐dried, annealed Åheim dunite (grain size of 400 μ ${upmu }$m) designed to elucidate the microphysical mechanism of transient dislocation creep during postseismic relaxation. We demonstrate that the transient mechanical behavior of dunite matches well with predictions of an intragranular model that involves the evolution of dislocation density and long‐range interactions among dislocations. Extrapolation to upper‐mantle conditions illustrates that this model can also explain the 1–2 orders of magnitude reduction in upper‐mantle viscosity that is inferred from studies of postseismic creep. We highlight opportunities to refine both intra‐ and intergranular models of transient dislocation creep.