We have investigated the magnetic properties of a photoresponsive organic–inorganic hybrid system, (SP-R)[Fe II Fe III (dto) 3 ] (SP=spiropyran; R=Me, Et, Pr; dto=C 2 O 2 S 2 ), under hydrostatic pressures. At ambient pressure, (SP-Et)[Fe II Fe III (dto) 3 ] and (SP-Pr)[Fe II Fe III (dto) 3 ] undergo one and two ferromagnetic phase transitions corresponding to the high-temperature phase with the spin configuration of Fe II (S=2)–Fe III (S=1/2), while (SP-Me)[Fe II Fe III (dto) 3 ] undergoes two ferromagnetic phase transitions corresponding to the coexistence of the high-temperature phase and the low-temperature phase with the spin configuration of Fe II (S=0)–Fe III (S=5/2). Under hydrostatic pressures, (SP-R)[Fe II Fe III (dto) 3 ] (R=Et, Pr) undergo a new ferromagnetic phase transition corresponding to the low-temperature phase caused by the pressure-induced charge transfer phase transition.