Sulfonated poly(ether ether ketone) (SPEEK) is a potential polymer for replacing Nafion in vanadium redox flow batteries (VRBs). However, at a high degree of sulfonation, SPEEK displays high swelling, poor mechanical stability, and high vanadium crossover. In this study, to improve membrane performance, composite membranes of SPEEK and finely ground microporous AMH-3 (G-AMH-3) are prepared with various G-AMH-3 contents and investigated. The physicochemical and mechanical properties, vanadium permeability, and VRB single cell performance of these SPEEK/G-AMH-3 composite membranes are evaluated using various characterization techniques. Interactions between SPEEK and G-AMH-3, and the permselective property of G-AMH-3, result in the composite membranes exhibiting good mechanical properties and low vanadium crossover. Optimal composite membranes gave a VRB that produced a higher charge–discharge capacity, higher cell efficiency, and better capacity retention than that using Nafion. These results indicate that SPEEK-based composite membranes with improved membrane performance, lower vanadium crossover, and good single cell performance were successfully prepared by incorporating G-AMH-3.