Co-pyrolysis of the seaweed Sargassum and polystyrene was investigated as a potential source of renewable energy. Sargassum is a brown macroalgae, posing a large disposal problem for beaches worldwide, and polystyrene is the plastic least recycled in the U.S. Although macroalgae bio-oil cannot be directly used as a result of a high oxygen content and low heating value, co-pyrolysis of macroalgae with low-oxygen-content waste polystyrene can enhance oil quality. Pyrolysis of pure Sargassum was conducted to determine the temperature producing the highest percent oil (600 degrees C). Co-pyrolysis of four different mixture ratios of Sargassum and polystyrene (5, 15, 25, and 33% by weight) was then conducted at 600 degrees C. Feedstocks and pyrolysis products (liquid oil and water, gas phase, and solid phase) were characterized using elemental analysis, thermogravimetric analysis, gas chromatography, surface area and adsorption isotherm analysis, and nuclear magnetic resonance. Co-pyrolysis with polystyrene improved the quality and quantity of the oil compared to pyrolysis of Sargassum alone. The oil quantity increased, from 3% for Sargassum alone to 29% for a mixture of 67% Sargassum and 33% polystyrene. Co-pyrolysis improved the oil potential heating value and decreased its potential for producing air pollution when combusted, by lowering its nitrogen content. Co-pyrolysis produced a gas with up to 7% hydrogen and 30% methane, which can be burned as a fuel. Co-pyrolysis of Sargassum and polystyrene, therefore, shows promise as a method for generating fuel and reducing disposal problems.