Social Anxiety Disorder (SAD) symptoms demonstrate a marked persistence over time, but little is known empirically about short-term processes that may account for this long-term persistence. In this study, we examined how self-reported and physiological stress reactivity were associated with persistence of SAD symptoms from early to late adolescence. A community sample of 327 adolescents (56% boys, Mage=13.01 at T1) reported their SAD symptoms for 6 successive years and participated in a public speaking task, during which self-reported (i.e., perceived nervousness and heart rate) and physiological (i.e., cortisol and heart rate) measures of stress were taken. Overall, our results point to a developmental process in which adolescents with a developmental history of higher SAD symptoms show both heightened perceived stress reactivity and heart rate reactivity, which, in turn, predict higher SAD symptoms into late adolescence.