Introduction:There is very little insight into the psychosocial characteristics of young adults susceptible to waterpipe tobacco use and their reactions to messages about harms of waterpipe tobacco smoking (WTS). We investigated how young adults who were or were not susceptible to WTS differed on various characteristics and their reactions to messages about WTS harms. Methods:Young adults ages 18 to 30 who had never used waterpipe tobacco were recruited through an online crowdsourcing site. Participants were stratified on susceptibility status (susceptible or not) and randomized to receive messages about harms and addictiveness of WTS or a control condition that received no messages. Participants' perceptions of risk and worry, their attitudes toward, and willingness/curiosity to try WTS were assessed. Results:Compared to nonsusceptible participants, susceptible participants perceived themselves to be at lower risk and worried less about harms and addictiveness of WTS, had more positive attitudes toward use, and expressed a greater willingness and curiosity to try it. Among susceptible participants, messages decreased willingness/curiosity to try WTS; messages had no effect on nonsusceptible participants. The message effects among susceptible participants were explained by more negative attitudes and less ambivalence toward WTS. Conclusions:Susceptible young adults' psychosocial characteristics place them at high risk for future uptake of WTS. Brief public health messages about harm and addiction may deter susceptible young adults' willingness to try WTS and prevent WTS initiation and progression. Implications:Findings suggest that in order to curb the initiation of WTS among susceptible young adults, interventions should target risk appraisals and attitudes toward WTS.