This study aimed to investigate the effect of putrescine on the immune function and intestinal bacteria of weaning piglets. Twenty-four male castrated weaning piglets on their 21st day were randomly assigned into four groups: control (basal diet) and treatment groups given basal diets supplemented with 0.05%, 0.1%, and 0.15% putrescine for 11 days. Results were as follows: (1) Dietary putrescine increased the villus height, width, height/crypt depth and surface area, and decreased the diarrhea index (P < 0.05). (2) Dietary putrescine increased the lysozyme and acid phosphatase activities and the amount of immunoglobulin M, antibacterial peptides, and transforming growth factor β1, but decreased the mRNA levels of tumor necrosis factor α, interleukin-6, interleukin-8 and inducible nitric oxide synthase (P < 0.05). (3) Dietary putrescine increased the mRNA expression of the mammalian target of rapamycin, signal transducer and activator of transcription, and Janus kinase 2 but decreased the mRNA expression of nuclear factor-kappa B P65 (P < 0.05). (4) Dietary putrescine increased the population of total bacteria, Lactobacillus, and Bifidobacterium and decreased that of Escherichia coli in the colon and cecum (P < 0.05). (5) Finally, dietary putrescine increased the concentrations of butyrate and total volatile fatty acids in the colon and those of acetate, propionate, and total volatile fatty acids in the cecum (P < 0.05). Overall, putrescine can enhance intestinal development, improve immune functions, and regulate the population of intestinal bacteria in weaning piglets.