C-phycocyanin is a photosynthetic pigment extracted from microalgae with great biotechnological potential due to their intense colors, fluorescence, and bioactive properties. Is the only blue pigment in the FDA food coloring agent exempt product list and can be used in a wide range of food products. Typically, phycocyanin is extracted from Spirulina, but low biomass yields due to the autotrophic lifestyle limits the market consolidation. Galdieria sulphuraria, an extremophile red microalga, phycocyanin producer that grow in high temperatures, acidic pH, and in different growth mode, appears as an alternative to produce this pigment. G. sulphuraria has a strong economic advantage because of its high yield per unit, however, the optimal conditions to high production of biomass and metabolites are currently poorly understood. Hence, in this study, we took CN ratio as the starting point to optimize the heterotrophic cultivation conditions of G. sulphuraria CCMEE 5587 for phycocyanin production. The experiments were carried out in bioreactors operating on batch mode with 800 mL of Cyanidium media supplemented with 10 g/L of glucose and different concentrations of (NH4)2SO4, to achieve CN ratios of 5, 10, 20, and 30. The other experimental conditions were: initial cell concentration of 100 mg/L, temperature of 42 °C, pH of 2.0, aeration of 1VVM, and absence of light. Considering that phycocyanin is an intracellular product, the best cultivation condition should combine high biomass and pigment productivity. Low CN ratios favored both biomass and phycocyanin production. For biomass, the highest productivity (1.5 g/L.d) was found in CN10, about 50% higher than that found in CN30. As expected, higher phycocyanin content (13.6 mg/g) was observed in the condition with the highest nitrogen concentration (CN5), where higher phycocyanin productivity (20.2 mg/L.h) was also found. Thus, decreasing C/N ratio by adding nitrogen can effectively increase the phycocyanin and biomass production.