Separators or electrolyte membranes are recognized as the key components to guarantee ion transport in rechargeable batteries. However, the ever‐growing applications of the battery systems for diverse working environments bring new challenges, which require advanced battery membranes with high thermal stability, excellent mechanical strength, high voltage tolerance, etc. Therefore, it is highly desirable to design novel methods/concepts to solve the current challenges for battery membranes through understanding the mechanism of novel phenomena and electrochemical reactions in battery systems working under unconventional conditions. Recently, the new emerging Janus separators or electrolyte membranes with two or more distinct chemical/physical properties arising from their asymmetric structure and composition, are promising to address the above challenges via rational design of their targeted functionalities. To this end, in this review, we first briefly cover the current challenges of the traditional battery membrane for battery devices working in unconventional conditions. Then, the state‐of‐art developments of the rational design of Janus membranes to overcome the above challenges for diverse battery applications are summarized. Finally, we outline these latest developments, challenges, and future potential directions of the Janus membrane. Our review is aimed to provide basic guidance for developing functional separators or electrolyte membranes for advanced batteries.