AbstractStimuli‐responsive aggregation of polymer chains in water has found a variety of applications in polymer science, biology, and chemical engineering. To date, the majority of the phase transitions between the aggregated and dissolved forms has been observed by changing the solution temperature, and an active and precise control on the phase transition with a high time resolution has been challenging. Herein, a reversible phase transition of poly(allylamine‐co‐allylurea) (PAU) in an aqueous electrolyte is achieved by electrochemical redox cycling of hexacyanoferrate(II/III) ([Fe(CN)₆]^4−/3−) ion pair. The aggregation and dissolution cycle can be completed in a high‐resolution time frame of as short as 5 s. The strong electrostatic interaction between the protonated primary amino group of PAU and the tetravalent [Fe(CN)₆]⁴⁻ anion induces the aggregation, while the oxidation to the trivalent [Fe(CN)₆]³⁻ anion reduces the attractive force, and the polymer chain redissolves in solution. The ureido group of PAU helps the chain‐folding process through the formation of inter/intrachain hydrogen‐bonding networks, resulting in the sharp phase transition. By using [Fe(CN)₆]^4−/3− as the electron mediator, the electrochemical control on the large transparency change of polymer aqueous solution is realized for the first time.