Organic additives are widely used in order to improve the performance of hydrotreating catalysts. Yet, the use of polymers, in particular bio-polymers, is less widespread. We present, herein, a strategy to coat alumina support with a biopolymer, namely polydopamine (Pdop). The Pdop coated supports provide a higher number of adsorption sites for Mo species compared to the bare support. The hydrotreating catalyst prepared on Pdop coated support showed improved performances for various hydrotreating reactions (aromatic hydrogenation, gas-oil HDS and HDN). The increase in activity is ascribed to a better accessibility of active sites, and the presence of carbon adlayer between the active phase and alumina limiting detrimental direct metal support interactions. Modification of the alumina surfaces by Pdop creates new adsorption sites for metallic precursors, thereby allowing an improved accessibility of active sites especially at high surface densities.