Demands for electromagnetic interference (EMI) shielding materials with small thickness, lightweight, and flexibility have been increasing to satisfy rapid advances in flexible and wearable electronics. Various EMI shielding materials have been explored and improved to meet these requirements for assist in the miniaturization of electronic devices, but is still a challenging task. In this work, we describe a simple and effective strategy for the fabrication of flexible nanoporous silver (Ag) membranes with unprecedented high effectiveness for EMI shielding. The nanoporous Ag membranes were fabricated by sequential vacuum filtration of a dispersion containing bacterial cellulose and Ag nanoparticles, which were followed by a hot-pressing process. The membrane with nanoporous Ag thickness of 1.2 μm exhibits high EMI shielding effectiveness (SE) of -53 dB (99.9%) in 0.5 - 18.0 GHz and an unprecedented high absolute EMI SE of 85,462 dB·cm2 g-1, which could be attributed to the nanoporous structures leading to an increasing of multiple reflections of the electromagnetic waves. In addition, the nanoporous Ag membrane showed highly stable electro-mechanical durability and fast heat dissipation properties, enabling desired EMI shielding performance for the wide range applications.