Uranium (U) speciation was investigated in anoxically preserved porewater samples of a natural mountain wetland in Gola di Lago, Ticino, Switzerland. U porewater concentrations ranged from less than 1 μg/L to tens of μg/L, challenging the available analytical approaches for U speciation in natural samples. Asymmetrical flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry allowed the characterization of colloid populations and the determination of the size distribution of U species in the porewater. Most of the U was associated with three fractions: <0.3 kDa, likely including dissolved U and very small U colloids; a 1–3 kDa fraction containing humic-like organic compounds, dispersed Fe, and, to a small extent, Fe nanoparticles; and a third fraction (5–50 nm), containing a higher amount of Fe and a lower amount of organic matter and U relative to the 1–3 kDa fraction. The proportion of U associated with the 1–3 kDa colloids varied spatially and seasonally. Using anion exchange resins, we also found that a significant proportion of U occurs in its reduced form, U(IV). Tetravalent U was interpreted as occurring within the colloidal pool of U. This study suggests that U(IV) can occur as small (1–3 kDa), organic-rich, and thus potentially mobile colloidal species in naturally reducing wetland environments.