A unique delivery system to reversibly and controllably load and release proteins under physiological conditions is desirable for protein therapeutics. We fabricate an ultrafast exponentially growing nanoporous multilayer structure comprised of two weak polyelectrolytes, poly(ethyleneimine) and alginate with thickness and chemical composition controlled by the assembly pH. For the first time, the assembled multilayered structure demonstrates stimuli-free reversible protein loading and release capability at physiological conditions by a synthetic material. The protein loading and release time can also be controlled by the assembled bilayer number. The highest loading capacity for the target protein and longest release time of proteins for layer-by-layer films reported to date have been achieved with a 15-bilayered film fabricated in this work. The prominent properties of the assembled film provide great potential for various biomedical applications, especially as a delivery system for protein therapeutics.