This study investigated the effects of incorporating fermented feed into the post-weaning piglet diet, with a focus on its impact on the growth, health, serum markers, immune function, diarrhea rate, and microbial community of post-weaning piglets. Forty-two post-weaning piglets (Duroc × [Landrace × Yorkshire], castrated male), with good health and similar weight (11.40 ± 1.36 kg) at 40 days of age, were randomly assigned to two groups. Throughout the trial, one group received a standard diet (control group) while the other one was given a diet supplemented with fermented feed (experimental group), respectively. The findings revealed that compared to the control group, the post-weaning piglets fed with fermented feed showed increased daily weight gain. The fermented feed also led to an increase in the height of villi in the duodenum or jejunum of post-weaning piglets (p < 0.05), accompanied by an improved ratio of villus height to crypt depth in the experimental group (p < 0.05). Furthermore, the fermented feed enhanced the richness and diversity of the intestinal microbial community in post-weaning piglets (p < 0.05), particularly increasing the relative abundance of Lactobacillaceae and Bifidobacteriaceae. These findings demonstrate that incorporating the fermented feed into the diet can reduce the diarrhea rate, improve growth performance, enhance immune function, and promote intestinal development in post-weaning piglets.