Background: Stigma and trust influence how adolescents seek support for mental illness, though it is unclear how these influence their decisions to approach a range of potential sources of support. Moreover, even less is known about the ways in which these issues are related when a friend discloses symptoms of mental illness. Objective: The study's aims were to understand the role of stigma, trust, and threat appraisals in adolescents' support seeking when exposed to their own, or to a friend's, symptoms of mental illness. Method: A vignette-based study comparing reports of support (friends, parents, teachers, professionals, and online) was completed with reference to either (i) experiencing symptoms of mental illness or (ii) having a friend disclose these types of symptoms. Two hundred and fifty adolescents (M = 12.75 years) answered questions pertaining to stigma (public and self), trust levels, threat appraisals, and support seeking. Results: When dealing with their own symptoms, threat accounted for 4.8 and 2.5% of the variance when seeking support from parents and professionals, respectively. Self-stigma accounted for 2.4% of variance when seeking support from parents and 0.8% of variance when seeking support from professionals. Trust moderated the association between threat and the use of online support. When responding to a friend's disclosure, higher levels of public-stigma were associated with lower support seeking from friends, parents, and professionals. Conclusions: This study showed a distinction in how adolescents deal with their own or a friend's symptoms of mental illness, and what resources they choose to ask for support from. Self-stigma, threat, and trust levels were particularly relevant when experiencing their own symptoms, while dealing with a friend's disclosure was related to levels of public-stigma.