BACKGROUND: Although highly efficacious psychological treatments for posttraumatic stress disorder (PTSD) exist, there is evidence that first-line psychological treatment approaches leave a substantial subgroup of patients still suffering from clinically relevant PTSD symptoms posttreatment. AIMS: We aimed to meta-analytically establish the prevalence and predictors of nonresponse to first-line guideline-recommended psychological treatments for PTSD. MATERIALS AND METHODS: This meta-analysis was preregistered (CRD42023368766). We searched the PTSD Trials Standardized Data Repository, Embase, Medline, PsychINFO, and PTSDpubs. We included randomized controlled trials (RCT), reporting data on nonresponse operationalized by (lack of) symptom reduction in PTSD symptoms at posttreatment of first-line guideline-recommended PTSD treatments for adult patients meeting criteria for a PTSD diagnosis. All studies published by October 10, 2023, were included. Data were extracted by two independent reviewers. We estimated the pooled average nonresponse rates and ORs. Subgroup and metaregression analyses targeting the nonresponse rates served to identify significant predictors. All analyses were conducted using three-level multilevel models. Study quality was assessed using Cochrane's RoB 2 tool. RESULTS: Eighty six studies with 117 active treatment conditions and 7,894 participants were included in the meta-analysis. The weighted average nonresponse rate was 39.23%, 95% CI (35.08%, 43.53%). Nonresponse was less frequent in the treatment condition compared to the control condition ( = 0.22). Subgroup analyses and metaregression revealed the type of analysis, population, type of intervention, treatment format, year of publication, age, sex, PTSD symptom severity, comorbid depression, and baseline depression score as significant predictors. The heterogeneity between studies was substantial to considerable (  = 83.12%). Half of the studies had a high risk of bias. CONCLUSIONS: This meta-analysis found that a substantial subgroup of patients suffering from PTSD still showed clinically significant symptoms after having received treatment. Treatment modifications should be considered for specific subgroups of PTSD patients based on predictors found to be associated with nonresponse.