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Simple Summary: Pelteobagrus fulvidraco is a common freshwater fish, mainly distributed in China, Vietnam, Laos, and southeast Siberia of Russia. Its meat is delicious and has high nutritional value. The purpose of the study was to explore the suitable farming density of P. fulvidraco in integrated rice–fish farming. Our study revealed that the stocking density of P. fulvidraco in a paddy field should not exceed 250 g/m2. High stocking density inhibited growth performance and caused physiological response. These insights offer guidance for the selection of an appropriate stocking density for P. fulvidraco in integrated rice–fish farming systems. Pelteobagrus fulvidraco is a freshwater fish commonly raised in rice fields, yet the optimal stocking density for this species remains unknown. Therefore, this study aimed to investigate the appropriate stocking density of P. fulvidraco in integrated rice–fish farming systems. Three different stocking densities––low density (LD, 125 g/m2), middle density (MD, 187.5 g/m2), and high density (HD, 250 g/m2)––were set up to evaluate P. fulvidraco's growth performance, stress indices, immune function, antioxidant status, and lipid metabolism after 90 days of farming. The results indicated that HD treatment had a detrimental effect on P. fulvidraco's growth parameters. HD treatment led to an increase in cortisol (Cor) and lactate (La) levels, but a decrease in glucose (Glu) content in serum. After 90 days of farming, an immune response accompanied by the increase of complement 3 (C3), C4, and immunoglobulin M (IgM) was observed in the HD group. Meanwhile, HD treatment induced oxidative stress and altered antioxidative status evidenced by the levels of catalase (CAT), glutathione peroxidase (Gpx), glutathione (GSH), malondialdehyde (MDA), superoxide dismutase (SOD), and total antioxidant capacity (T-AOC) in serum or liver. Additionally, the lipid metabolism-related genes including lipoprotein lipase (lpl), peroxisome proliferators-activated receptor (pparα), carnitine palmitoyltransferase-1 (cpt-1), and sterol regulatory element binding protein-1 (srebp-1) were markedly downregulated in the HD and/or MD group after 90 days of farming. In conclusion, this study contributes to a better understanding of P. fulvidraco's response to different stocking densities in integrated rice–fish farming systems. We suggest that the appropriate stocking density for P. fulvidraco in these farming systems should be below 250 g/m2, considering both fish growth and physiological responses. [ABSTRACT FROM AUTHOR]