•The Janus nanofibrous membrane was successfully fabricated by a bottom-up electrospinning strategy.•The nanofibrous membrane exhibited robust asymmetric wettability.•The Janus nanofibrous membrane could efficiently separate various oil–water emulsions driven solely by gravity. Superwettable membranes with superhydrophilicity/underwater superoleophobicity (SHL/UWSOB) or hydrophobicity/underoil superhydrophobicity (HB/UOSHB) are considered critical components in the chemical industry. However, despite the unique wettability of these membranes, there are still challenges in meeting the needs of the developing society. Herein, a Janus nanofibrous membrane with a special micro-nanostructure was constructed by a bottom-up electrospinning strategy using two different precursors: zeolitic imidazolate framework-8-polyacrylonitrile/N,N-dimethylformamide (ZIF-8-PAN/DMF) and candle soot-polyacrylonitrile/N,N-dimethylformamide (CS-PAN/DMF). The PAN@ZIF-8 nanofibrous membrane exhibited HB/UOSHB, with water contact angles higher than 150° in oils, whereas the PAN@CS nanofibrous membrane displayed favorable SHL/UWSOB, with oil contact angles greater than 151° in water. Simultaneously, the resulting Janus nanofibrous membrane exhibited excellent rejection rate (99.1%) towards various oil–water emulsions and a high flux of up to 1720 ± 20 L m−2h−1 driven only by gravity. The as-prepared Janus membrane was cycled five times without compromising the flux and rejection rate, demonstrating its satisfactory cycling stability. Comparative analysis demonstrated that the comprehensive separation performance of the resulting Janus nanofibrous membrane was superior to that of current homogeneous or other Janus membranes. More importantly, the as-prepared Janus membrane exhibited favorable chemical stability under harsh conditions (acidic and alkaline environments). This study presents a facile and efficient method for constructing a performance-stabilized Janus membrane, which can be utilized for multiple applications, such as oil–water separation, osmotic gasification, and seawater desalination.