The backscattering properties of four marine phytoplankton from three size-classified taxa at different chlorophyll concentrations were directly measured in the laboratory using a Hydroscat-6 instrument. The ancillary parameters measured included the absorption and scattering coefficients, chlorophyll-a concentration, cell size, and cell concentration. We found that the value of the backscattering coefficients at each band increased with an increase in chlorophyll concentration for each alga, and the spectral shape at the blue bands were changed for the picophytoplankton and nanophytoplankton while it remained the same for microphytoplankton with changes in the chlorophyll-a concentration. We also found that the backscattering variation range changed with the increase in the chlorophyll concentration: the larger the cell size, the smaller its range of change. In addition, the smaller particles had relatively higher backscattering at shorter wavelengths, but no relationship between the cell size and contribution to the backscattering was found. Moreover, we also found a relationship between size and both the backscattering ratio and backscattering cross-section, in accordance with previous results. The results from this work provide a good foundation for improving the accuracy of identifying red tide algae using water color remote sensing.