Reaction between mid‐ocean ridge basalt (MORB) and crystal mush in the lower oceanic crust has been invoked to explain chemical variations of both MORB and minerals in the lower oceanic crust. Nonetheless, such reactions have been little studied experimentally. We conducted experiments to investigate the mechanisms and chemical consequences of melt‐mush interaction by reacting molten MORB with troctolite at 0.5 GPa. Isothermal experiments demonstrate that melt infiltrates into troctolite with dissolution of plagioclase and olivine. The reacted melts have higher MgO and Al2O3 and lower TiO2 and Na2O contents and crystallize more primitive olivine and plagioclase compared to those crystallized from the unreacted melts, suggesting melt‐mush reaction could result in the formation of high‐Al basalt. The melt compositional variations induced by reaction also significantly affect the calculated pressures for MORB fractionation, indicating that major element‐based barometers for MORB fractionation can only be used reliably if reaction can be ruled out. After reaction, the troctolite contains olivine with plagioclase inclusions and poikilitic clinopyroxene with partially resorbed olivine and plagioclase chadacrysts, indicating that melt‐mush interaction occurs through dissolution‐reprecipitation mechanisms. Clinopyroxene has high Mg# (>83) and elevated Na2O and TiO2 contents, and olivine has different Fo versus Ni correlations from fractional crystallization models, which provide testable parameters for the effect of melt‐mush reaction in the rock record. By comparison with samples from lower oceanic crust and layered intrusions, we propose that melt‐mush reaction plays an important role during magma transport in the crystal mush in both oceanic and continental magma systems. Plain Language Summary Magmas erupted at mid‐ocean ridges represent the largest volcanic output from the Earth's interior and have long been recognized as a probe to mantle composition and melting history. During its ascent from the mantle, magma crystallizes minerals in crustal magma reservoirs. Although crystallization modifies magma compositions, this process is well‐understood and can therefore be corrected for. However, when crystallization proceeds, a network of interconnected minerals forms with small amounts of magma between them. Recent studies show that magma compositions might also be modified by reaction with minerals in this so‐called crystal mush. Our study, for the first time, performed experiments to explore how such reactions work and how they change the compositions of both magma and minerals. The results show that during reaction, magma can dissolve some minerals and crystallize others, which changes the magma compositions. Such variations in magma compositions challenge our understanding on the histories and depth of magma crystallization derived from them and eventually affect our understanding of mantle composition. We also found that minerals in the mush carry distinct chemical signatures after reaction, which can be used as tracers for melt‐mush reaction in nature. Comparison with natural mineral data suggests that such reaction is common in magma systems. Key Points Melt‐mush reactions proceed through diffusion‐assisted dissolution and reprecipitation Reaction significantly shifts melt major element compositions, indicating it can limit the application of major‐element barometers for MORB Clinopyroxene Mg#‐Na‐Ti and olivine Fo‐Ni relationships in cumulates provide tracers for melt‐mush reaction in nature