In this work, the electrochemical nucleation and growth mechanism of cobalt onto a polycrystalline silver electrode was investigated to analyze its initial stages of deposition in aqueous media. Electrodeposition was carried out using an aqueous solution containing 0.01 M CoSO₄ and 1 M Na₂SO₄ at pH 7.0 and 25 °C. Cyclic voltammetry study revealed that cobalt deposition is an irreversible process controlled by Co²⁺ mass transport. Current density–time transients obtained from potentiostatic steps were analyzed using the Scharifker–Hills and Hermann–Tarallo models. The results showed that electrodeposition follows a three-dimensional progressive nucleation mechanism with diffusion-controlled growth, in which active sites are continuously created during the process. Kinetic parameters, such as the nucleation rate, density of active sites, and saturation density, increased markedly at more negative cathodic potentials. From the transient analysis, the Co²⁺ diffusion coefficient was calculated as 3.3 × 10⁻⁶ cm² s⁻¹. Furthermore, analysis based on atomistic nucleation theory yielded a critical nucleus size of zero and a Gibbs free-energy barrier of 1.54 × 10⁻²⁰ J nucleus⁻¹, suggesting favorable thermodynamic conditions for cobalt nucleation on polycrystalline silver.