Dyes contained in wastewater are one type of the organic pollutants that adversely affects the environment. Biochar is known for effectively removing dyes from dye-containing wastewater. Biochar has been widely researched as environmentally friendly and inexpensive technology to remove dyes from wastewater. Biomass, which is used as a raw material for making biochar is very diverse, including natural materials, industrial wastes, and agricultural by-products. It also has different properties, and biomass type is a parameter to determine adsorption capabilities of biochars for the organic contaminant. This study used kelp to make biochar among marine macroalgae generated in large quantities around coastal areas, and developed biochar-based nano-composites using Mg and Fe oxides. The characteristics of the manufactured biochars were investigated using FTIR, SEM, and BET surface area analyzer. The value of pH and initial dye concentration were used as experimental variables for the evaluation of biochar adsorption capacities for dyes. In addition, various adsorption kinetics and isothermal models were used to evaluate the dye removal capabilities of biochars. The nano-composites showed a huge difference in ash content and pH compared to the conventional biochars. The nano-composites which had chemically-modified surface affected the adsorption of dyes. In particular, the maximum adsorption capacity of KB-Mg for congo red (CR) was 1003.464 mg/g. For crystal violet (CV), KB which was not pre-treated showed the highest adsorption capacity. These results demonstrated that the nano-composite with Mg was effective in removing the anionic dye, and the cationic dye was well adsorbed to pristine biochar which was not specifically treated. It is considered that the main mechanism for the adsorption of both CR and CV to the biochars is an electrostatic interaction.