The three‐dimensional polyallylamine (PAH)‐functionalized gold nanodendrites (PAH‐3D‐Au‐NDs) were successfully synthesized through a simple, one‐step electrodeposition method. SEM, TEM, energy‐dispersive spectrum mapping, and X‐ray photoelectron spectroscopy measurements were used to characterize the morphology, structure, and composition of PAH‐3D‐Au‐NDs. The PAH‐3D‐Au‐NDs were then used as the functional interface to effectively immobilize the ruthenium(III) ion through coordination interactions between the ruthenium(III) ion and PAH. The obtained RuIII/PAH‐3D‐Au‐NDs/Au electrode showed excellent electrocatalytic activity towards iodate reduction. With the use of PAH‐3D‐Au‐NDs, the proposed iodate electrochemical sensor displayed a fast response (less than 3 s), wide linear range (4.99×10−8 to 8.50×10−4 M), and low detection limit (1.66×10−8 M).