A total of 84 pigs (17.6 ± 2.8 kg initial BW at 6 wk of age) were used in a 40-d trial to evaluate effects of dietary supplemental β-mannanase (400 U/kg feed, CTCBIO Inc., Seoul, Korea) on growth performance, ileal nutrient digestibility and gut health of nursery pigs. Pigs were allotted to 2 treatments (14 pens per treatment, 3 pigs per pen) based on a randomized complete block design with sex and initial BW as blocks. Experimental diets were corn (phase 3: 50%, phase 4: 57%), soybean meal (phase 3: 27%, phase 4: 20%), and 20% DDGS based with or without 400 U β-mannanase/kg feed. Growth performance (ADG, ADFI, and G:F) was recorded weekly. Plasma was collected on d 35 to quantify tumor necrosis factor-α (TNF-α) and malondealdehyde (MDA). On d 35, 0.3% titanium oxide was added as an indigestible marker to the diets for an additional 4 d feeding. On d 40, 16 pigs (1 pig per pen, 8 pens per treatment) were euthanized to collect digesta from jejunum, ileum, and colon (to measure viscosity and pH value) and to collect tissues from duodenum, jejunum and ileum (for morphology, TNF-α, and MDA evaluation). Supplementation of β-mannanase reduced (P < 0.05) viscosity of jejunal digesta (2.52 to 1.97 cP, respectively). Supplementation of β-mannanase improved (P < 0.05) G:F during phase 3 (0.593 to 0.617) and overall period (0.572 to 0.589), pH of colon digesta (5.99 to 6.33), and ileal digestibility of NDF (31.3 to 41.1%) and ADF (26.8 to 38.7%). Supplementation of β-mannanase increased (P < 0.05) ileal villus height (377 to 432 µm), decreased (P < 0.05) jejunal crypt depth (249 to 212 µm), and improved (P < 0.05) villus height:crypt depth ratios in jejunum (1.58 to 2.10) and ileum (1.65 to 2.02). Supplementation of β-mannanase tended to reduce (P = 0.078) TNF-α concentration (7.94 to 6.46 pg/mg) in jejunal mucosa. Collectively, dietary β-mannanase (400 U/kg) could improve feed efficiency by decreasing digesta viscosity and increasing nutrient digestibility, and could also maintain gut health by improving gut morphology and reducing inflammatory response.