Abstract: Under intensive aquaculture conditions, bacterial diseases frequently occur in the large yellow croaker (Larimichthys crocea). The development of safe and effective plant-derived feed additives represents an important approach for reducing antibiotic use and promoting antibiotic-alternative strategies in aquaculture systems. The objective of this study was to evaluate the regulatory effects of dietary supplementation with Silphium perfoliatum on growth performance, immune function, and intestinal microecology of L. crocea. Powdered S. perfoliatum was incorporated into a basal diet at supplementation levels of 3%, 5%, and 7%, respectively. Juvenile L. crocea were fed the experimental diets continuously for seven weeks. Growth performance indicators, including body length, body weight, and feed conversion ratio, were determined after the feeding trial. Resistance to bacterial infection was evaluated through challenge experiments using Pseudomonas plecoglossicida and Vibrio harveyi, and survival performance was assessed accordingly. Digestive- and immune-related enzyme activities were measured in the intestine, spleen, and liver. In parallel, the expression levels of immune-related genes were analyzed in the same tissues. In addition, changes in intestinal microbial community structure were investigated based on 16S rRNA gene sequencing. Dietary supplementation with S. perfoliatum at levels of 3% and 5% significantly promoted increases in body length and body weight of L. crocea (P < 0.05), among which the 5% supplementation group showed the most pronounced growth-promoting effect. Feed conversion ratio was significantly reduced in all supplementation groups ranging from 3% to 7% (P<0.01), indicating improved feed utilization efficiency. In the bacterial challenge experiments, fish fed the diet supplemented with 5% S. perfoliatum exhibited enhanced resistance against both P. plecoglossicida and V. harveyi. The relative percent survival values reached 53.85% and 60.00%, respectively, and mortality was significantly reduced compared with the control group. Biochemical analyses demonstrated that dietary supplementation with 5% S. perfoliatum significantly increased the activities of lysozyme, superoxide dismutase, and lipase in the intestine, spleen, and liver (P < 0.05). Gene expression analysis further showed that the transcription levels of immune-related genes, including immunoglobulin M (IgM), interleukin-1β (IL-1β), and cluster of differentiation 4 (CD4), were significantly upregulated in fish fed the 5% supplemented diet (P < 0.05). Analysis of intestinal microbiota revealed that dietary supplementation with 5% S. perfoliatum significantly increased microbial species richness. Moreover, the dominant bacterial genus in the intestine shifted from Acinetobacter to beneficial genera, including Lactobacillus, Paracoccus, and Demequina, indicating a marked alteration of intestinal microbial composition. In conclusion, dietary supplementation with 5% S. perfoliatum synergistically promotes growth performance, enhances resistance to bacterial infection, and improves intestinal microecology in L. crocea. This research provides scientific evidence supporting the application of S. perfoliatum as a plant-derived feed additive for promoting healthy aquaculture of L. crocea.