Photosynthesis is the driving force behind the growth of seaweeds. During the photosynthetic process, seaweeds capture energy from sunlight and use this energy to convert dissolved carbon dioxide (CO2) into sugars. These are ultimately converted into building blocks for the seaweed body, such as proteins, fatty acids, and polysaccharide fibers. Effective photosynthesis is dependent on the efficient balance between light energy capture and its conversion into chemical energy and sufficient CO2 uptake from the surrounding aqueous environment. This means that photosynthesis is influenced by fluctuations in factors such as day–night cycles, water motion, and water chemistry, as well as biotic factors such as the presence of competing algae species. Seaweeds have evolved genetic-based mechanisms for acclimation to their habitats. The core components of the photosynthesis reactions are found in all known seaweeds. However, red seaweed species have evolved unique mechanisms for light energy harvest and conversion. Due to their high commercial value, understanding the photosynthetic machinery of red seaweed species is of great importance for efficient cultivation. This chapter aims to review the photosynthetic traits of the most commercially important red seaweed genera—Gracilaria, Porphyra/Pyropia, and Kappaphycus/Eucheuma. It describes the mechanisms of light harvest and CO2 capture for the seaweeds and the main findings regarding light cultivation conditions and CO2 acquisition strategies. From this information, conclusions are drawn suggesting best practices for the cultivation of these seaweeds, especially in terms of light regimes and water motion.