Purpose: While opioids are part of usual care for analgesia in the intensive care unit (ICU), there are concerns regarding excess use. This is a systematic review of non-steroidal anti-inflammatories (NSAIDs) use in critically ill adult patients. Methods: We conducted a systematic search of MEDLINE, EMBASE, CINAHL, and Cochrane Library. We included randomized control trials (RCTs) comparing NSAIDs alone or as an adjunct to opioids for analgesia. The primary outcome was opioid utilization. We reported mean difference for continuous outcomes and relative risk for dichotomous outcomes with 95% confidence intervals (CIs). We evaluated study risk of bias using the Cochrane risk of bias tool and evidence certainty using GRADE. Results: We included 15 RCTs (n=1621 patients). Adjunctive NSAID therapy to opioids reduced 24-hour oral morphine equivalent consumption by 21.4mg (95% CI: 11.8-31.0mg reduction, high certainty) and probably reduced pain scores (measured by visual analogue scale) by -6.1mm (95% CI: -12.2 to +0.1, moderate certainty). Adjunctive NSAIDs probably had no impact on duration of mechanical ventilation (-1.6 hours, 95% CI: -0.4 to -2.7 hours, moderate certainty) and may have no impact on ICU length of stay (-2.1 hours, 95% CI: -6.1 to +2.0 hours, low certainty). Variability in reporting of adverse outcomes (e.g. gastrointestinal bleeding, acute kidney injury) precluded their meta-analysis. Conclusion: In critically ill adult patients, NSAIDs reduced opioid use, probably reduced pain scores, but were uncertain for duration of mechanical ventilation or ICU length of stay. Further research is required to characterize the prevalence of NSAID-related adverse outcomes.