Alkaline polymer electrolyte fuel cells (APEFCs) are a new class of electrochemical devices that intrinsically enable the use of nonprecious metal catalysts. As an important component of APEFCs, alkaline polymer electrolytes (APEs) have been a research focus in recent decades. To minimize the ohmic loss and to facilitate the water transport, the APE membrane should be as thin as possible, which generally requires a trade-off between the ionic conductivity and the mechanical robustness/dimensional stability of the membrane. Here, we report a new reinforced APE membrane that can effectively disentangle such a trade-off. The quaternary ammonia poly(ether ether ketone) (QAPEEK) membrane is highly conductive but suffers from the overuptake of water, which leads to significant membrane swelling and weak mechanical strength. Upon reinforcing with sulfonated nanobamboo fiber (s-NBF), the swelling degree decreases from 27.5 to 7.5% in 80 °C water. The thickness of such an s-NBF/QAPEEK membrane can then be reduced to 15 μm, which diminishes the electrical resistance, very suitable for APEFC applications.