Uranium contamination is an environmental issue in Europe and worldwide, particularly in mining-impacted areas. Pentavalent uranium is a key intermediate in U biogeochemistry, challenging the traditional view of U reduction as a transition from U(VI) to U(IV). This work explores microbial U(VI) reduction in weakly U-contaminated mine water (1 mg/L) using glycerol as an electron donor. 16S rRNA sequencing showed that glycerol biostimulation enriched fermentative (Propionibacteriaceae, Propionivibrio) and sulphate-reducing (Desulfobulbus, Desulfovibrio) bacteria, contributing to direct or indirect U(VI) reduction. Advanced spectroscopic and microscopic analyses confirmed reduction of U(VI) to U(IV) and U(V), with U(IV) forming biogenic uraninite nanoparticles. Remarkably, U(V) was identified in two forms: FeU(V)O₄ nanoparticles and U(V)-carbonate complexes. This study is the first report of a stable biogenic U(V)-phase for four weeks under oxic conditions, which is resistant to reoxidation and provides insights into U(VI) reduction, the role of U(V) species in biogeochemical processes, and U bioremediation strategies.