Thiourea (TU) adsorption on Ag(111) from aqueous solutions was investigated by in situ scanning tunneling microscopy operating under potential control. Hexagonal arrangements with d = 0.44, 0.38, and 0.33 nm were imaged at potentials close to -1.2 V (vs SCE), where conjugated voltammetric current peaks are observed. The analysis of in situ Fourier transform infrared reflection-absorption spectra (FT-IRRAS) shows that these current peaks mainly involve electroadsorption/electrodesorption of TU on the Ag(111) surface. Data from ex situ X-ray photolectron spectroscopy of TU-covered Ag(111) closely resemble those obtained for adsorbed alkanethiols on the same substrate, suggesting that the canonical form of TU is the species adsorbed from aqueous solutions. Experimental evidence of TU degradation into other sulfur species is also observed.