To explore the application potential of insect-derived functional microorganisms in short-cycle leafy vegetable production, we evaluated the effects of Serratia marcescens BRC-CXG2, isolated from larvae of Monochamus alternatus, on romaine lettuce in a pot experiment. Plant growth traits, biomass accumulation, nutritional quality, endogenous hormones, and rhizosphere microbial communities were systematically evaluated. The results demonstrated that inoculation significantly promoted seedling development. Plant height and root length increased by 48.7% and 29.1%, respectively, while shoot and root dry weights were 1.78- and 1.85-fold higher than those of the control. Vitamin C and total sugar contents increased by 76.4% and 98%, respectively. The levels of gibberellins (GA₃)-, indole-3-acetic acid (IAA)-, and abscisic acid (ABA)-immunoreactive equivalents increased by 1.5-, 1.29-, and 1.75-fold. High-throughput 16S rDNA gene and ITS amplicon sequencing further revealed that inoculation reshaped the composition of bacterial and fungal communities in the rhizosphere. Collectively, these findings demonstrate that insect-derived S. marcescens exhibits significant growth-promoting potential in short-cycle leafy vegetable systems, with effects associated with hormone regulation, enhanced total sugar accumulation, and shifts in rhizosphere microbial community structure.