The study focused on the effect of an extended photoperiod on the radiation use efficiency (RUE) by the tomato transplants (Solanum lycopersicum L.) in the pre–reproductive period. In two consecutive series of experiments, the photoperiod was 16 and 22 hours. The photon irradiance at the plant tops was maintained at low, medium and high levels: 100, 170 and 240 μmol m–2 s–1, respectively. The plants were grown under two lighting systems with different light quality. The difference was 7% higher blue flux share in Spectrum II. The use of an extended photoperiod, especially in combination with high irradiance level, resulted in the plant leaf chlorosis. When varying the radiation dose components, the deviation from the reciprocity law was recorded. By the analysis results, the chlorophyll degradation was a response to the extended photoperiod rather than the radiation dose. Without additional blue flux, under a regular photoperiod, RUE reduced by 8% at the high irradiance level. Under extended photoperiod, the shift from the low to high irradiance level reduced RUE by 20–37%, with bigger reduction values being observed at higher irradiance levels. Seven percent addition of blue flux made it possible to increase RUE by 5–8% at the same and lower irradiance levels and under the regular photoperiod. With the extended photoperiod under these conditions, RUE decreased by 8–21%. The study results verify a great influence of an extended photoperiod on RUE, while the degree of influence depends on other parameters of light environment – light quality and irradiance level.