Heterogeneity in demographic and outcomes data with corresponding measurement instruments (MI) creates barriers for data pooling and analysis. Several core outcome sets developed in inflammatory bowel disease (IBD) homogenise outcomes data. A parallel Minimum Data Set (MDS) for baseline characteristics is lacking. We conducted a systematic review to develop the first MDS. A systematic review of observational studies from 3 databases (2000 to 2021). Titles and abstracts were screened; full-text articles reviewed, and data extracted by two reviewers. Baseline data were grouped into 10 domains: demographics, clinical features, disease behaviour/complications, biomarkers, endoscopy, histology, radiology, healthcare utilisation and patient-reported data. Frequency of baseline data and MI within respective domains are reported. From 315 included studies (600,552 subjects), most originated from Europe (196; 62%), and North America (59; 19%), and were published between 2011 and 2021 (251; 80%). The most frequent domains were demographics (311; 98.7%) and clinical (289; 91.7%); 224 (71.1%) studies reported on the triad of sex (306; 97.1%), age (289; 91.7%) and disease phenotype (231; 73.3%). Few included baseline data for radiology 19; 6%), healthcare utilisation (19; 6%) and histology (17; 5.4%). Ethnicity (19; 6%), race (17; 5.4%) and alcohol/drug consumption (6; 1.9%) were least reported demographics. From 25 MI for clinical disease activity, Harvey Bradshaw Index (n=53) and Mayo score (n=37) were most frequently used. Substantial variability exists in baseline population data reporting. These findings will inform a future consensus for MDS in IBD to enhance data harmonisation and credibility of real-world evidence.