Tomato protection from diseases is necessary to obtain high yields of quality fruits. In protected soil conditions, tomatoes bear fruit for up to 265 days, while the fruits are harvested 1 or 2 times a week starting from 60 days after seed germination for early varieties and 100 days for late varieties. When growing tomatoes, especially during the fruiting period, it is optimal to use biological pest control agents, which, unlike chemical ones, are harmless to humans and do not accumulate in the fruits. Existing biological products are not effective enough against the entire range of tomato diseases. It is necessary to look for new strains of microorganisms. This paper presents the results of the study of strain 23B78/1 Serratia plymuthica with the aim of exploring the prospects for its use as a biocontrol agent. Pesticides based on this species are not registered in the Russian Federation. Species identification was determined by 16S gene sequence analysis and biochemical profiling. Antagonistic activity against phytopathogenic fungi was assessed in vitro using double culture method. Phytotoxicity testing was carried out on germinating seeds of tomato. Evaluation of antagonistic effect revealed effectiveness against the following phytopathogenic fungi: Alternaria solani, Botrytis cinerea, Colletotrichum truncatum, Fusarium citri, F. incarnatum, F. duofalcatisporum, F. incarnatum, F. oxysporum, Globisporangium ultimum, Sclerotinia sclerotiorum. The maximum antagonistic effect was observed during paired fusion with fungus B. cinerea, mycelium of which grew strictly in the opposite direction from the bacterium. Germination of tomato seeds in presence of strain 23B78/1 did not reveal any inhibitory effect on seed germination and development of young tomato plants. The conducted research shows that the strain Serratia plymuthica 23B78/1 is promising for creation of biofungicide for protecting tomato plants.