Young adults' perceptions and use of heated tobacco products (HTPs) are understudied. This mixed methods study analyzed (i) Fall 2020 survey data from 2470 US young adults (mean[subscript age] = 24.67; 19.5% and 25.2% past-month cigarette and e-cigarette use; 4.1% ever HTP use) assessing HTP use intentions and perceptions (1 = not at all to 7 = extremely) and (ii) Spring 2021 interview data regarding IQOS (most widely available HTP) in a subset of 40 e-cigarette users. Among survey participants, HTPs versus cigarettes and e-cigarettes showed lower use intentions (mean = 1.27 vs. mean = 1.73, mean = 2.16) but were perceived as less addictive (mean = 5.11 vs. mean = 6.28, mean = 5.82) and harmful (mean = 5.37 vs. mean = 6.65, mean = 5.62). HTPs were perceived more socially acceptable than cigarettes but less than e-cigarettes (mean = 3.39 vs. mean = 3.13, mean = 4.37). Among interviewees, most reported limited HTP knowledge. A few perceived IQOS as a hybrid of traditional cigarettes and e-cigarettes. Most perceived IQOS as harmful but less harmful than cigarettes and were uncertain in relation to e-cigarettes. Over half reported minimal interest in trying IQOS; common reasons included IQOS containing tobacco, limited flavors and use complexity. The varied perceptions of IQOS versus cigarettes and e-cigarettes underscore the need for continued surveillance of perceptions, use and marketing of IQOS to inform regulatory oversight and potential interventions.