Magnetic nanoparticles enable an effortless catalyst recovery at the end of a chemical reaction, and therefore contribute to more sustainable and greener industrial processes. Herein we report the synthesis of magnetically retrievable Ag NPs for the catalytic hydrogenation and oxidation of aldehydes in water. Upon brief exposure to microwave irradiation, Ag and Fe3O4 NPs formed directly within carboxymethyl cellulose (CMC), an inexpensive, biobased polymer support. The catalyst was tested for the hydrogenation of aldehydes in water, showing a high activity and selectivity to C═O bonds against C═C bonds. Aldehydes can also be oxidized to the corresponding carboxylic acid with the same catalyst, even with substrates that do not self-oxidize under air. The organic matrix and the magnetic support allowed the catalyst to be recycled up to five times with negligible metal leaching in the reaction medium.