The crystal and magnetic structures of La0.75Ca 0.25MnO3 manganite are studied under high pressures up to 4.5 GPa in the temperature range 12-300 K by the neutron diffraction method. At normal pressure and temperature TC = 240 K, a ferromagnetic state is formed in La0.75Ca0.25MnO3. At high pressures P ≥ 1.5 GPa and at temperatures T < TN ≈ 150 K, a new A-type antiferromagnetic state appears. A further increase in pressure leads to an increase in the volume fraction of the antiferromagnetic phase, which coexists with the initial ferromagnetic phase. The effect of high pressure causes a considerable increase in TC with the slope dTC/dP ≈ 12 K/GPa. Calculations performed in the framework of the double exchange model with allowance for the electron-phonon interaction make it possible to explain this pressure dependence of TC on the basis of experimental data.