In the search of useful environmentally friendly fiber reinforced plastics, this is the first study that evaluates the capability to use natural pine rosin (in pure or maleated glycerol ester form) as a biodegradable coupling agent. Polypropylene as polymer matrix and discarded agave fiber (Agave tequilana) as reinforcing agent at different concentrations, were coupled with each one of the two rosins above mentioned; a commercial maleated polypropylene (Epolene, E-43) agent was used to compare their effect. As generic reference, composites without coupling agent were also tested. Mechanodynamic and mechanostatic tests clearly show an increment in mechanical properties of the composites, using any of the 3 coupling agents. The results obtained with maleated rosin were similar or slightly better than the ones obtained with the commercial agent for composites with high fiber content. Fiber content and coupling agent effect on composites crystallinity,was evaluated by differential scanning calorimetry. In addition, water absorption as a function of time was followed to evaluate the effect of surface modification, and FTIR analysis allowed the observation of the fiber-polymer matrix interaction that was promoted with the coupling agents. The effect of such interaction obtained with the different coupling agents, was observed by scanning electron microscopy. The results show the feasibility to use the natural pine rosin in pure or modified form as biodegradable coupling agents.