Purely organic crystals, κ-X₃(Cat-EDT-TTF)₂ [X = H or D, Cat-EDT-TTF = catechol-fused tetrathiafulvalene], are a new type of molecular conductor with hydrogen dynamics. In this work, hydrostatic pressure effects on these materials were investigated in terms of the electrical resistivity and crystal structure. The results indicate that the pressure induces and promotes hydrogen (deuterium) localization in the hydrogen bond, in contrast to the case of the conventional hydrogen-bonded materials (where pressure prevents hydrogen localization), and consequently leads to a significant change in the electrical conducting properties (i.e., the occurrence of a semiconductor–insulator transition). Therefore, we have successfully found a new type of pressure-induced phase transition where the cooperation of the hydrogen dynamics and π-electron interactions plays a crucial role.