Tomato is a widely cultivated and economically important crop worldwide. This study aimed to test the effect of light spectra used in indoor cultivation on the growth, biochemical profile, and resistance of Solanum lycopersicum ‘Bawole Serce’ seedlings against Alternaria alternata, Alternaria solani and Botrytis cinerea. During the phase of first leaf emergence, the seedlings were transferred to a semi-sterile growth room with a controlled environment (20°C, 18-h photoperiod, 50 μmol · m⁻² · s⁻¹ PPFD, 65% RH) for a 3-week cultivation period. Five light treatments differing in red/blue (R/B) light ratio were tested: I (LED tube; R/B 5.55), II (fluorescent tube; R/B 0.72), III (fluorescent tube; R/B 1.19), IV (LED panel; R/B 0.51), V (LED panel; R/B 0.20). The best parameters in terms of shoot length, shoot fresh and dry weights, and number of leaves were obtained in treatment I, in contrast to IV and V. Plants from treatments IV and V had the smallest leaf area, perimeter, vertical length, and horizontal width. As for the root system, the highest fresh weight, area, length of the longest root, total length, and the number of root tips and forks were found in treatments I and II. The least developed root systems were observed in IV and V. The greatest chlorophyll, carotenoids and anthocyanins accumulation was enhanced by treatment II. Treatments I−III stimulated the biosynthesis of phenolic compounds. The highest superoxide dismutase activity was detected in plants from treatments I and II. As for A. alternata and A. solani, the level of disease symptoms was significantly higher for treatments IV and V than for I-III. The highest/lowest level of B. cinerea infection was found in treatments II/I, respectively. The least susceptible to infection by all tested pathogens were leaves from treatment I. Light spectrum composition is of practical importance for tomato seedling production.