Agaricus blazei Murrill (AB) is a mushroom known for its medicinal applications. AB stipe (ABS) is a by-product of the mushroom’s processing, which is rich in bioactive substances, such as polysaccharides. This experiment was conducted to assess the effects of dietary ABS supplementation on growth performance, short-chain fatty acid production, antioxidant capacity, immune function, meat quality, gut function, and caecal microbiota in broilers. A total of 180 one-day-old male Arbor Acre broilers (46.17 ± 2.47 g) were randomised to 3 treatments, with 6 replicates per treatment and 10 broilers per replicate. The treatments included one basal diet (control group) and two experimental diets supplemented with 1% and 2% ABS, respectively. This trial lasted 42 days and was divided into Phase 1 (day 1–21) and Phase 2 (day 22–42). The results showed that broilers fed ABS exhibited a significant reduction in the feed conversion ratio throughout the study period (P < 0.05), and tended to have increased average daily gain in Phase 2 (P < 0.1) and the overall period (P < 0.1). On day 42, dietary ABS supplementation significantly decreased the concentration of interleukin-1β (P < 0.05) and total cholesterol in serum (P < 0.05). Moreover, ABS addition significantly increased the villus height-tocrypt depth ratio (P < 0.05) in the jejunum and tended to increase villus height (P < 0.1) and decrease crypt depth (P < 0.1). In addition, the ABS-supplemented groups showed higher microbial diversity and increased proliferation of beneficial bacteria in the caecum. Based on these findings, it can be concluded that ABS supplementation can improve growth performance, anti-inflammatory activity, and intestinal function.