Selenium (Se) is an essential trace element for humans and animals with a narrow window between deficiency and toxicity levels. Application of conventional chemical Se fertilizers to increase the Se content in crops could result in secondary soil and water contamination due to the low utilization rate of Se. Therefore, it may be beneficial to produce slow-release Se-enriched organic fertilizers locally from Se-containing (waste)water. This may contribute to the worldwide drive for resource recovery and circular economy. In this study, we aim to evaluate the bioavailability of Se released from two Se-enriched biomaterials (duckweed and anaerobic sludge) in soil, and to assess the potential of these two biomaterials as Se fertilizers. Green beans were grown on sandy and loamy soils fertilized with Se-enriched duckweed and anaerobic sludge at rates equivalent to 1 and 5 mg Se/kg soil. Non-planted treatments served as controls. The mobility and solubility of Se were assessed by measuring the Se concentration in the soil pore water and in the different bean tissues. The Se composition in the Se-enriched duckweed and anaerobic sludge was analyzed by HPLC-ICP-MS and EXAFS (Extended X-ray Absorption Fine Structure), respectively. Results showed that Se was mainly present as the hexavalent selenate form in duckweed and the zerovalent form in anaerobic sludge. The different Se species in the two biomaterials resulted in distinct Se release patterns. After 3 d of incubation, the application of 1 and 5 mg Se/kg soil of duckweed increased the Se concentration in pore water to 537 and 4375 µg/L for sandy soil, and 413 and 1238 µg/L for loamy soil, respectively; whereas the sludge amendment of 1 and 5 mg Se/kg soil led to an increment of Se content in pore water to 65 and 322 µg/L for sandy soil, and 72 and 387 µg/L for loamy soil, respectively. However, increasing the incubation time from 3 to 42 d reduced the Se content in soil pore water by 92% for sandy soil and 89% for loamy soil in non-planted treatments amended with 5 mg Se/kg soil of duckweed. The decrease of bioavailable Se followed a second-order equation. By contrast, the Se concentration in the pore water of soils supplied with sludge remained stable during the entire incubation period. The application of Se-enriched duckweed and anaerobic sludge significantly increased the Se concentration in the different tissues of beans (seeds, leaves, stems, and roots), which was supported by the positive correlation between Se in the soil pore water and Se in the beans tissues. In addition, the Se concentration in beans fertilized with sludge was 1 to 3 times higher than in those amended with duckweed. The seeds of beans produced in this study could contribute to achieving the recommended daily Se intake for human diets. Overall, Se-enriched green beans were successfully produced after amending the soil with Se-enriched duckweed and anaerobic sludge, demonstrating their potential as organic Se-rich fertilizers for Se deficient regions.