Abstract Background delirium is an acute state of confusion that affects >20% of hospitalised patients. Recent literature indicates that more severe delirium may lead to worse patient outcomes and health system outcomes, such as increased mortality, cognitive impairment and length of stay (LOS). Methods using systematic review methodology, we summarised associations between delirium severity and patient or health system outcomes in hospitalised adults. We searched MEDLINE, EMBASE, PsycINFO, CINAHL and Scopus databases with no restrictions, from inception to 25 October 2018. We included original observational research conducted in hospitalised adults that reported on associations between delirium severity and patient or health system outcomes. Quality of included articles was assessed using the Newcastle–Ottawa Scale. The level of evidence was quantified based on the consistency of findings and quality of studies reporting on each outcome. Results we included 20 articles evaluating associations that reported: mortality (n = 11), cognitive ability (n = 3), functional ability (n = 3), patient distress (n = 1), quality of life (n = 1), hospital LOS (n = 4), intensive care unit (ICU) LOS (n = 2) and discharge home (n = 2). There was strong-level evidence that delirium severity was associated with increased ICU LOS and a lower proportion of patients discharged home. There was inconclusive evidence for associations between delirium severity and mortality, hospital LOS, functional ability, cognitive ability, patient distress and quality of life. Conclusion delirium severity is associated with increased ICU LOS and a lower proportion of patients discharged home. Delirium severity may be a useful adjunct to existing delirium screening to determine the burden to health care system resources.