BACKGROUND: Use of high-flow nasal oxygen (HFNO) for treatment of adults with acute respiratory failure (ARF) has increased. PURPOSE: To assess HFNO versus noninvasive ventilation (NIV) or conventional oxygen therapy (COT) for ARF in hospitalized adults. DATA SOURCES: English-language searches of MEDLINE, Embase, CINAHL, and Cochrane Library from January 2000 to July 2020; systematic review reference lists. STUDY SELECTION: 29 randomized controlled trials evaluated HFNO versus NIV (k = 11) or COT (k = 21). DATA EXTRACTION: Data extraction by a single investigator was verified by a second, 2 investigators assessed risk of bias, and evidence certainty was determined by consensus. DATA SYNTHESIS: Results are reported separately for HFNO versus NIV, for HFNO versus COT, and by initial or postextubation management. Compared with NIV, HFNO may reduce all-cause mortality, intubation, and hospital-acquired pneumonia and improve patient comfort in initial ARF management (low-certainty evidence) but not in postextubation management. Compared with COT, HFNO may reduce reintubation and improve patient comfort in postextubation ARF management (low-certainty evidence). LIMITATIONS: Trials varied in populations enrolled, ARF causes, and treatment protocols. Trial design, sample size, duration of treatment and follow-up, and results reporting were often insufficient to adequately assess many outcomes. Protocols, clinician and health system training, cost, and resource use were poorly characterized. CONCLUSION: Compared with NIV, HFNO as initial ARF management may improve several clinical outcomes. Compared with COT, HFNO as postextubation management may reduce reintubations and improve patient comfort; HFNO resulted in fewer harms than NIV or COT. Broad applicability, including required clinician and health system experience and resource use, is not well known. PRIMARY FUNDING SOURCE: American College of Physicians. (PROSPERO: CRD42019146691).