  External pressure and associated changes in lattice structures are key to
realizing exotic quantum phases such as high-$T_{\rm c}$ superconductivity.
While applying external pressure is a standard method to induce novel lattice
structures, its impact on organic molecular crystals has been less explored.
Here we report a unique structural phase transition in (DMET-TTF)$_2$AuBr$_2$
under pressure. By combining advanced high-pressure techniques and $ab$
$initio$ calculations, we elucidate that (DMET-TTF)$_2$AuBr$_2$ undergoes a
transition from a quasi-one-dimensional lattice to a nearly perfect rectangular
lattice at 0.9 GPa. This transition leads to the realization of an
antiferromagnetic Mott insulator with $T_{\rm N}=66$ K, the highest $T_{\rm N}$
in low-dimensional molecular crystal solids to date. Upon increasing the
pressure, the antiferromagnetic ordering is suppressed, and a superconducting
phase with $T_{\rm c}=4.8$ K emerges around 6 GPa. Our study reveals the
significant impact of external pressure on lattice structures of organic
molecular crystals and highlights the intricate relationship between
geometrical frustration and superconductivity. Our findings also pave the way
for realizing functional organic molecular crystals through changes in lattice
structures by pressure.
