Demand for the development of non-antibiotic growth promoters (AGP) in animal production surged in recent years. However, elucidating the specific mechanisms and action of prebiotics, probiotics, and synbiotics as non-AGP in animals is still in progress. This work investigated and compared faecal microbiotas of weaned piglets under the administration of a basal diet (CON) and with prebiotic lactulose (LAC), probiotic Enterococcus faecium NCIMB 11181 (PRO), or their synbiotic combination (SYN). Although prebiotics and/or probiotics in the diet significantly increased alpha-diversity compared with CON values, no differences were detected in richness and diversity values among the treatment groups (LAC, PRO, and SYN). At phylum level, the Firmicutes to Bacteroidetes ratio increased in the treatment groups in comparison to the CON group, and the lowest abundance of Proteobacteria was found in LAC group. At family level, Enterobacteriaceae decreased in all treatments, especially more than 10-fold reduction in LAC group compared with CON group. At genus level, the highest abundance of Oscillibacter was detected in PRO group, the highest Clostridium in LAC group, and the highest Lactobacillus in SYN group; the abundance of Escherichia was lowest in LAC group. Clustering in the DAPC plots illustrated distinct separation of the feeding groups (CON, LAC, PRO, and SYN) from one another, showing that microbial communities had different compositions according to different feed additives. Effects of LAC and PRO treatments on the faecal microbiota suggest independent mechanisms; nonetheless, the impact of SYN might also be distinct from that when each are administered singly as LAC or PRO.