Green biorefineries aim to sustainably produce chemicals, materials, proteins and energy by processing green biomass, such as grass, into a solid fraction (fibers) and a liquid fraction (green juice) for further refining. Here, we propose to incorporate microalgae cultivation in the green biorefinery concept to obtain a higher protein production from green juice obtained from grass. A mixed culture of green microalgae was cultivated on multiple dilutions and after different pre-treatments of green juice from agricultural grass. After 19 days, 1 g/L of algal biomass was reached in a 10% dilution pre-treated by sedimentation and pH adjustment to 8. Further treatments to reduce the microbial load in the grass juice did not increase algal productivity. Overall, these findings offer new perspectives for protein production in a green biorefinery.