Developing effective and sustainable alternatives to traditional petroleum plastics is one of the primary problems faced by the scientists and engineers of this generation. Red algae-based polymer is one such alternative, as it is sourced from an abundant and renewable natural resource. However, as with many natural plastic alternatives, its mechanical properties are generally inferior to petroleum plastics, and so additives need to be added to compensate. This project's aim is to determine the effects of various amounts of natural sisal fiber additives on plasticized red algae polymer composite films. We have successfully extracted algae polymers from locally sourced red algae, identified the polymer type, cast films with 50 wt% glycerol plasticizer and varying levels of fiber as reinforcement, and tested the mechanical and thermal properties of these composite films. The concentration of sisal in the samples varied from 0 to 45% and each treatment underwent tensile, DSC, and TGA analysis. From these tests it has been determined that sisal fiber is an effective reinforcement for red algae polymer films, showing increasing thermal stability with higher sisal fiber weight percentages and optimal mechanical properties between 15-30 fiber wt%.