To study the effects of diet complexity and multicarbohydrase (MC) supplementation, 144 piglets (6.70 ± 0.81 kg of BW) weaned at 21 ± 2 d of age (1:1 male to female ratio) were assigned to 1 of 6 dietary treatments in a 3 × 2 factorial arrangement based on diet complexity (complex1, complex2, and simple) and MC addition (0 vs. 0.1% of MC). Diets were provided in a 2-phase feeding program with phase I (d 1 to 14) and phase II diets (d 15 to 28). Complex1 was formulated to mimic a conventional weaner diet with blood plasma, fish meal, dried whey, and skim milk powder, whereas complex2 partially or totally replaced these ingredients with various plant-based ingredients. The simple diet primarily comprised corn, wheat, and soybean meal. No interactions were found between diet complexity and MC supplementation ( > 0.10), except for apparent total tract digestibility (ATTD) of ash and globulin concentration on d 28 ( < 0.05). Pigs fed the complex1 diet had greater ( < 0.01) ADG, ADFI, and G:F than those fed the other diets during the first week after weaning. However, no differences in ADG or ADFI were observed for pigs fed the complex1 and simple diets throughout the 28-d experimental period ( > 0.10), whereas the complex2 diet led to lower ADG and ADFI compared with the complex1 diet ( < 0.05). Overall, G:F was greater for pigs fed the simple diet compared with those fed the complex1 diet ( < 0.01). Pigs fed the simple diet showed ATTD of DM, OM, GE, and ash comparable to those fed the complex1 diet on d 14 ( > 0.10). In contrast, greater ATTD of DM, OM, NDF, GE, and ash was observed ( < 0.05) in pigs fed the complex1 diet than in those fed the complex2 diet or the simple diet on d 28. The simple diet increased the lymphocyte proportion in serum compared with the complex1 diet ( < 0.01) on d 14. Pigs fed the complex1 diet had a greater ( < 0.05) ratio of villus height (VH) to crypt depth (CD) in the ileum compared with the other diets. The complex2 diet decreased ( < 0.05) the fecal score during 3 wk of the postweaning period compared with the complex1 diet. Dietary MC supplementation increased ( < 0.05) G:F during the overall experimental period, VH in the ileum, VH:CD ratio in the duodenum, and ATTD of DM and GE but it reduced fecal score ( < 0.05). In conclusion, feeding a simple diet resulted in BW comparable to feeding the complex1 diet, but it led to inferior intestinal morphology and ATTD of nutrients on d 28. Also, MC supplementation could be beneficial regardless of diet complexity.