Summary Background Intestinal bacteria produce metabolites and by‐products necessary for homeostasis. Imbalance in this equilibrium is linked to multiple pathologies including inflammatory bowel disease (IBD). The role of the gut microbiota in determining treatment response is becoming apparent, and may act as biomarker for efficacy. Aim To describe knowledge about the intestinal microbiota on disease severity and treatment outcomes in IBD Methods Descriptive review using PubMed to identify literature on the intestinal microbiota in IBD Results Severe IBD has a less diverse microbiota with fewer commensal microbiota communities and more opportunistic pathogenic bacteria originating from the oral cavity or respiratory tract. IBD treatments can alter gut microbiota composition, but in vitro/in vivo studies are needed to prove causation. A diversification of the microbiota is observed during remission. Patients with a more diverse baseline microbiome and higher microbial diversity show better response to anti‐tumour necrosis factor‐α, vedolizumab and ustekinumab therapy. Higher abundance of short chain fatty acid‐producing bacteria, fewer mucus‐colonising bacteria and lower abundance of pro‐inflammatory bacteria have also been associated with a favourable outcome. Predictive models, based on a combination of microbiota, clinical data and serological markers, have good accuracy for treatment outcome and disease severity. Conclusion The intestinal microbiota in IBD carries a set of promising biomarkers of disease activity and prediction of therapeutic outcome. Current insights may also help in designing microbiota modulation strategies to improve outcomes in IBD.