For the first time, ZnSe epitaxial layers grown by molecular‐beam epitaxy are shown to exhibit large contrast, low power, and extremely long‐term stable thermally induced absorptive optical bistability. It is observed over a wide temperature range between 169 K and RT, with a strongly variable loop width and a switch‐back adjustable to be less than 50% of the respective switch‐down value. Critical slowing down as well as switch‐down times are studied in dependence of the excess over the switch‐down intensity values.