Transformation superplasticity is a deformation mechanism that occurs during the phase transformation of an externally stressed material. Pressure-induced transformation superplasticity, which has long been postulated to occur in olivine in the Earth's interior, is shown to take place in water ice. Pure ice specimens were subjected to hydrostatic pressure cycling between 0 and 300 MPa to reversibly induce the I/II transformation (at 220 or 230 K) or the I/III transformation (at 240 K). When a small uniaxial compressive stress was applied during cycling, the specimens exhibited a uniaxial compressive strain (as large as 22% after a single cycle) proportional to the applied stress, in agreement with observations and theory for transformation superplasticity of metals and ceramics, induced by polymorphic thermal cycling. Additionally, specimens of ice containing 10 vol% SiO2 were deformed by this mechanism during the I/II transformation at 230 K. The presence of silica particles was found to enhance the deformation, rather than strengthen the ice. These experimental results are discussed both qualitatively and quantitatively in terms of existing models of transformation superplasticity and implications for the lithosphere rheology of the icy moons of the outer planets.