Abstract <p>The absence of growth photoinhibition in the diatom <i>Phaeodactylum tricornutum</i> over a wide range of high light intensities (172–688 μmol photons m^–2 s^–1) was demonstrated. Understanding the reasons for such high resistance is essential. For that purpose, the dynamics of short-term and long-term acclimation of this species, which was grown initially at a low light intensity (17.2 μmol photons m^–2 s^–1), were examined at two high light intensities (172 and 516 μmol photons m^–2 s^–1). Within 0.5 h since the beginning of the experiment, the non-photochemical quenching of chlorophyll <i>a</i> fluorescence (NPQ) increased. After 2 h, it reached a maximum value (1.04–1.14). Following this, a relaxation period occurred, accompanied by a decrease in this parameter. After a day, it increased again and then gradually decreased until the experiment was completed. At both light intensities, the maximal efficiency of photosystem II (<i>F</i>_v/<i>F</i>_m), the maximal rate of electron transport (ETRs), the proportion of active reaction centers (<i>RC</i>/<i>ABS</i>), and a performance index on absorption basis that assesses the efficiency of photosystem II (<i>PI</i>_ABS) changed slightly during photoacclimation. By the end of the second day, the content of chlorophyll <i>a</i>, chlorophyll <i>c</i>, and fucoxanthin, normalized to organic carbon, had lower initial values by 1.8–3.4 times. At the same time, the specific content of diadinoxanthin (DD) was 6 times lower than its initial value. Stabilization of most structural and functional parameters of the culture during the second day may indicate the completion of the photoacclimation. The absence of photosystem II photoinhibition in <i>P. tricornutum</i> under given light conditions is probably due to the high level of NPQ, which is several times higher than its values for other diatoms.</p>