Abstract In an effort to quickly impart flame retardant behavior to cotton fabric, and in ambient conditions, a water-soluble polyelectrolyte complex (WPEC) was evaluated. A OnePot mixture consisting of polyethylenimine and poly(sodium phosphate) imparts self-extinguishing behavior in a single step. This nanocoating maintains the fabric's weave structure by conformally coating individual fibers. Appreciable weight gain (23%) is achieved with just a single 30 s immersion in the WPEC suspension. Treatment of this coating with acidic buffer further renders it insoluble in water and durable to rinsing. Uncoated cotton is readily consumed during vertical flame tests, while OnePot-coated fabric, followed by buffer treatment of pH 5 or lower (16.5% weight gain or less), self-extinguishes through an intumescent mechanism. Microscale combustion calorimetry reveals a total heat release reduction of 88% and peak heat release rate reduction of 81%. This work demonstrates the ability of a WPEC nanocoating to prevent ignition of cotton fabric with few processing steps and relatively low weight gain.