Due to high consumption and non-renewable nature of fossil fuels, rapid development of potential renewable energies such as biofuel derived from microalgae is necessary for achieving the goals of sustainable growth and carbon neutrality. However, the high energy consumption required for microalgal biomass harvesting is regarded as a major obstacle for large-scale microalgal biofuel production. In the present study, the marine green microalgae Tetraselmis sp. was used to investigate a rapid and energy-efficient biomass collection method among different methods such as gravity sedimentation, auto-flocculation (at target pH), flocculation by polymers followed by magnetic separation, and centrifugation. The results showed that sufficient high cell densities of microalgae were obtained under the optimized growth conditions after 21 days of cultivation, and the microalgae could be easily flocculated and collected by magnetic separation using synthesized magnetic nanocomposites. The results also showed that among the different methods, magnetic separation was more efficient for biomass harvesting because of its simple and fast processing steps as well as low energy consumption. However, further investigation on different target microalgal species and their cultivation conditions, such as salinity and medium pH, will be required before application for large-scale biofuel production in the future.