There is a high demand for a volatile-organic-compound (VOC)-free approach to converting high performance polymers (HPPs), such as polyetherketoneketone (PEKK), to porous structures. This is particularly true for achieving flexible foams from these very rigid HPPs. This current work introduces a facile and environmentally benign approach to fabricating flexible PEKK foams via a hydration-induced foaming and desulfonation treatment. This foaming technique possesses three major advantages: energy efficiency, environment friendliness, and super flexibility. The resultant flexible PEKK foam has a uniform porous structure with a cell diameter of ca. 5 mu m, as well as low mass density of ca. 0.42 g/cm(3). Thermal analysis revealed that the porous PEKK showed a high thermal stability with a thermal degradation temperature of 520 degrees C, showing superiority over other reported flexible polymer foams. Compared with solid PEKKs fabricated by compression molding, the porous PEKK has a higher crystallinity and more thermodynamically stable crystal structures. According to nanoindentation analysis, the flexibility of porous PEKK may be a result of the better chain mobility in porous PEKK. The reduced modulus and hardness of porous PEKK are 0.73 +/- 0.16 and 0.09 +/- 0.03 GPa, respectively. The porous structures also efficiently reduced the dielectric constant from ca. 4.0 to 2.6 in a wide frequency range.