Polyimide (PI)–matrix composites reinforced by poly(p-phenylene benzobisoxazole) (PBO) microfibers and carbon nanofibers were prepared, and wear behaviors of the composites sliding against stainless steel in sea water were investigated. The results showed that the single incorporation of PBO microfibers or carbon nanofibers can significantly improve the wear resistance of PI–matrix composites. Further, a synergistic effect between PBO microfibers and carbon nanofibers was found. Namely, hybrid incorporation of the two fibers led to the best wear resistance of PI–matrix composites. During the friction and wear process, flexible PBO microfibers can effectively absorb and dissipate the friction energy; stiff carbon nanofibers can protect the PBO/PI interfaces and the PBO microfibers from mechanical damage.