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MgSiO₃ majorite is the most significant endmember of the Al-deficient majorite garnets that form at depths higher than ∼400-km, in the mantle transition zone (MTZ). Here we report elastic wave velocity measurements on polycrystalline MgSiO₃ majorite samples, up to 18.4 GPa and 2000 K by ultrasonic interferometry techniques combined with in situ X-ray diffraction measurements in a multianvil apparatus. Our data show MgSiO₃ majorite has the lowest elastic moduli among silicate garnet endmembers, under the pressure and temperature conditions of the MTZ. These new data combined with those of other garnet endmembers allowed to estimate V_P and V_S of majorite with compositions relevant to the MTZ. The results suggest that variation of MgSiO₃ component in garnet may play a role for interpreting seismic gradients atop the 410-km discontinuity while below ∼520 km, the velocity contrasts hold too low values to explain the discrepancy between mineral physics data and seismological observations.