Objective: Sudden gains during psychotherapy have been found to be predictive of positive treatment outcomes. Previous attempts at predicting occurrence of sudden gains have yielded equivocal findings. Recently, intraindividual variability in symptoms during treatment was suggested as a trans-therapeutic and trans-diagnostic predictor of sudden gains. The goal of the present study was to examine this predictor in Internet-delivered treatment for social anxiety disorder (SAD) and to examine whether this predictor predicts sudden gains when measured before treatment begins. Method: We examined data from a preregistered randomized controlled trial (RCT) of Internet-delivered cognitive-behavioral therapy (CBT) for SAD (n = 101). We measured variability in symptoms both within-treatment and before treatment (i.e. during waitlist). Results: Intraindividual variability in symptoms significantly predicted sudden gains both when measured before treatment or within-treatment and correctly classified 84% and 83% of individuals to sudden gains versus non-sudden gains status, respectively. Conclusions: Intraindividual variability in symptoms can predict sudden gains in Internet-delivered treatment for SAD, thus supporting its trans-diagnostic and trans-therapeutic nature. Predicting sudden gains before treatment begins has implications for treatment planning and clinical decision making as well as for personalized tailoring of interventions. (PsycInfo Database Record (c) 2020 APA, all rights reserved).