This study presents a technique for determining the true light absorption spectrum of a dense culture of the marine coccolithophore Chrysotila sp. using a single-beam MS 122A spectrophotometer equipped with an integrating sphere. The main problem with standard measurements is the distortion of the spectrum due to light scattering by cells, which is especially noticeable in the near-infrared region (750–800 nm), where the pigments are not absorbed, but the signal is not zero. To compensate for the scattering effect, the authors used an approach based on recording absorption spectra at two positions from the integrating sphere: standard (close to) and at a distance of 2 mm. The correction factor, independent of wavelength, was calculated from data in the range of 750–800 nm. Its value was 3.77. The true absorption spectrum, stripped of the scattering contribution, was calculated using the proposed formula. The technique has shown its effectiveness for cultures with a high cell density, providing zero absorption values in the near-infrared region. However, with a low cell concentration, the technique is inapplicable due to a significant increase in errors. Thus, the work demonstrates a practical way to correctly determine in vivo absorption spectra using available equipment, which is important for ecological and physiological studies of phytoplankton as well as for development of regional remote sensing algorithms.