Background: We investigated the occurence and effects of sudden gains in internet-based cognitive behavioral therapy (ICBT) for obsessive-compulsive disorder (OCD). As secondary analysis, we studied whether sudden gains are moderated by D-Cycloserine (DCS). Methods: Using data from a recent trial (N = 128), we assessed sudden gains based on weekly symptom change on the self-rated Yale-Brown Obsessive-Compulsive Scale (Y-BOCS). Treatment outcome was assessed using the clinician-administered Y-BOCS. We compared treatment outcomes for sudden gainers vs. non-sudden gainers and gradual gainers at post-treatment and 3, 12 and 24-month follow-up. Results: Thirty-eight percent of the patients experienced a sudden gain. Sudden gainers showed significantly larger improvements on the clinician-administered Y-BOCS than non-sudden gainers at post-treatment (d = 1.11), as well as 3-month (d = 1.06), 12-month (d = 0.88) and 24-month follow-up (d = 0.77). Sudden gainers also showed significantly less severe OCD symptoms than gradual gainers at post-treatment (d = 0.50), as well as 3-month (d = 0.55) and 12-month follow-up (d = 0.57). In addition, patients receiving DCS showed a significantly higher rate of sudden gains. Conclusions: Sudden gains are common in ICBT for OCD and are associated with favorable short and long-term treatment outcomes.