BACKGROUND: Despite the relevance of cognitive processes such as rumination, worry, negative metacognitive beliefs in emotional disorders, the existing literature about how these cognitive processes moderate the effect of treatment in treatment outcomes is limited. The aim of the present study was to explore the potential moderator effect of baseline cognitive processes-worry, rumination and negative metacognitive beliefs-on the relationship between treatment allocation (transdiagnostic cognitive-behavioural therapy -TD-CBT plus treatment as usual-TAU vs. TAU alone) and treatment outcomes (anxiety and depressive symptoms, quality of life [QoL], and functioning) in primary care patients with emotional disorders. METHODS: A total of 631 participants completed scales to evaluate worry, rumination, negative metacognitive beliefs, QoL, functioning, and anxiety and depressive symptoms. RESULTS: Worry and rumination acted as moderators on the effect of treatment for anxiety (b = -1.25, p = .003; b = -0.98, p = .048 respectively) and depressive symptoms (b = -1.21, p = .017; b = -1.34, p = .024 respectively). Individuals with higher baseline levels of worry and rumination obtained a greater reduction in emotional symptoms from the addition TD-CBT to TAU. Negative metacognitive beliefs were not a significant moderator of any treatment outcome. LIMITATIONS: The study assesses cognitive processes over a relatively short period of time and uses self-reported instruments. In addition, it only includes individuals with mild or moderate anxiety or depressive disorders, which limits generalization to other populations. CONCLUSIONS: These results underscore the generalization of the TD-CBT to individuals with emotional disorders in primary care with different cognitive profiles, especially those with high levels of worry and rumination.