This study investigated the effects of different light spectral qualities and temperature on the photosynthesis and pigment content of a subtidal edible red alga, Meristotheca papulosa. Photosynthesis–irradiance (P–E) experiments were carried out under red (660 nm), blue (450 nm), green (525 nm, light-emitting diodes), and white light (visible light, metal halide lamp), and at 12, 20, and 28 °C, respectively. Maximum net photosynthetic rates (NP_max) were highest under green light. Other P–E parameter estimates were similar among algae under red, blue, and green light, including their lower initial slope (α) and higher saturation irradiances (E_k) as compared to those under white light. Additionally, NP_max and E_k under white light were highest at 28 °C, and lowest at 12 °C with characteristic photoinhibition at irradiances greater than 150 μmol photons m⁻² s⁻¹. Photosynthesis–temperature (P–T) experiment revealed that the maximum gross photosynthetic rate (GP_max) occurred at 22.1 °C, which was within the optimal temperature range of F_v/F_m (21.5–23.6 °C). Exposures to the different light qualities at 100 μmol photons m⁻² s⁻¹ for 7 days showed increased phycoerythrin (PE) concentration of algae under blue and green light, while chlorophyll-a and phycocyanin (PC) showed little variation in all light qualities. Therefore, considering future management prospects for M. papulosa mariculture, we suggest that green light could be utilized to enhance photosynthesis. Furthermore, if the aim is to achieve high PE content for an improved reddish-color fresh product, exposure to blue or green light could be a good alternative.