The present study investigated whether supplementing fibre-degrading enzymes can ameliorate the severity of subclinical necrotic enteritis (NE) in broiler chickens offered wheat- or maize-based diets. A total of 1,544 mixed-sex broiler chickens were assigned to 16 experimental treatments as a 2 × 2 × 4 factorial arrangement of treatments. The factors were the following: NE challenge, yes or no; diet type, wheat- or maize-based; and enzyme supplementation, control (no enzyme), family 10 xylanase (XYN10), family 11 xylanase (XYN11) or β-mannanase (MAN). Each treatment was replicated 6 times, with 16 birds per replicate pen. A three-way challenge × diet type × enzyme interaction occurred for body weight at 21 d of age ( = 0.025) and overall feed conversion ratio ( = 0.001). In the non-challenged birds fed the wheat-based diet, supplementing MAN increased d 21 body weight compared to the control. In challenged birds fed the maize-based diet, supplemental XYN11 impeded body weight and overall FCR compared to the control. Birds offered the maize-based diet presented heavier relative gizzard weights at both 16 and 21 d of age ( < 0.001) and reduced liveability ( = 0.046) compared to those fed the wheat-based diet. Enzyme supplementation reduced ileal and jejunal digesta viscosity at 16 d of age only in birds fed the wheat-based diet ( < 0.001). XYN11 increased ileal digesta viscosity in birds fed the maize-based diet, and MAN reduced it in birds fed the wheat-based diet at 21 d of age ( = 0.030). Supplementing XYN11 improved ileal soluble non-starch polysaccharides (NSP) digestibility in birds fed the wheat-based diet compared to non-supplemented birds ( < 0.001). Birds fed the wheat-based diet displayed a higher abundance of and Enterobacteriaceae and butyric acid in the caeca at 16 d of age compared to birds fed the maize-based diet ( < 0.05). In conclusion, supplemental XYN11 exacerbated the negative impact of NE on growth performance in birds fed the maize-based diet. Supplementing wheat-based diets with fibre-degrading enzymes ameliorates production losses induced by NE.