The carbonization of Nannochloropsis gaditana microalgae biomass was found to produce bio-coal that is similar to bituminous coal used in thermal power plants. Currently, microalgae that capture CO2 while they are in the growth stage are considered a source for the production of biofuels. The carbonization of biomass for producing bio-coal has received attention for its ability to improve the biomass quality for producing solid biofuels. The research was focused on optimizing a fixed carbon index (FCindex), which allows finding operational conditions of carbonization to favor the fixed carbon content without significantly affecting the bio-coal yield. The optimization carried out by response surface methodology in a thermogravimetric analyzer allowed the prediction of optimal carbonization conditions to achieve an FCindex of 191% at 403 °C, 71 °C/min, and 60 min of residence time. The bio-coal produced under optimized conditions was characterized by 59% of fixed carbon and 41% of volatiles on a dry and ash-free basis, which is similar to bituminous coal. The promising results of dry carbonization producing bio-coal similar to bituminous coal could promote this technology, avoiding the necessity of hydrothermal carbonization. Because a high ash content was detected in the final product, further studies using the optimized conditions and a washing step should be conducted.