Insecure attachment has been suggested as a major risk factor for mental health problems as well as a key element for the development and trajectory of psychiatric disorders. The aim of this meta-analysis was to assess whether insecure attachment constitutes a global transdiagnostic risk factor in bipolar disorder, depression, and schizophrenia spectrum disorders. We conducted a PRISMA-based systematic quantitative review to explore the prevalence of insecure attachment among patients of three representative psychiatric disorders - major depression, schizophrenia spectrum disorders and bipolar disorder - in comparison with healthy controls (HC) from a transdiagnostic point of view. Effect sizes on differences of anxious, avoidant and insecure prevalence were calculated based on 40 samples including a total of n = 2927 individuals. Overall, results indicated a large effect on prevalence of insecure attachment across all disorders compared to HC (k = 30, g = 0.88, I² = 71.0%, p < 0.001). In a transdiagnostic comparison, the only difference was found in avoidant attachment, which was significantly lower (p = 0.04) compared to HC in the schizophrenia spectrum disorder subgroup (k = 10, g = 0.31, I² = 76.60%, p < 0.0001) than the depression subgroup subgroup (k = 12, g = 0.83, I² = 46.65%, p < 0.0001). The lack of further transdiagnostic differences between three distinct psychiatric disorders corroborates insecure attachment as a general vulnerability factor to psychopathology. Our findings warrant further investigations, which should explore the pathways from attachment insecurity towards psychopathology. Insecure attachment likely has implications on assessment, prediction and treatment of psychiatric patients.