High-flow nasal oxygen therapy (HFNO) is highly versatile and employed in varied situations, including after extubation, in cases of respiratory failure, and at the end of life. However, its impact on swallowing function is not yet elucidated. Therefore, this scoping review aimed to clarify how HFNO affects swallowing function and whether it poses a risk for aspiration pneumonia. We searched the databases MEDLINE via PubMed, CINAHL, Web of Science, and Cochrane Central Register of Controlled Trials (CENTRAL) from inception till June 5, 2024, to gather relevant studies. No language restrictions were applied. The eligibility criteria were as follows: (1) studies involving adults using HFNO, (2) studies examining swallowing function and the occurrence of pneumonia, and (3) excluding gray literature such as conference proceedings. A total of 1449 articles were initially identified, of which 12 that met the inclusion criteria were included in the final analysis. Of them, five involved healthy adults, whereas seven involved patients. Six studies investigated the effects of flow rate on the swallowing function, five studies on healthy adults, and one on patients. The review findings indicated that as the flow rate increased, the swallowing function was affected by the shortening of the latency time of the swallowing response and laryngeal vestibular closure time. Additionally, the increase in the flow rate caused modulation of the swallowing-breathing coordination. However, none of the studies reported that HFNO increases the incidence of pneumonia. The increased flow rates of HFNO affect the swallowing function; however, the actual impact on patients is currently unknown. This study involved a small number of healthy adults; therefore, further research based on the patient characteristics is warranted.