Background: The development of initiatives to improve access to psychological therapies has been driven by the realization that untreated anxiety and depression are both very common and costly to individuals as well as society. Effective and efficient treatments, mostly in the form of cognitive behavioural therapies (CBT), can be used in ways which enhance their acceptability and accessibility. To date, numbers of group therapies have been developed to improve cost efficiency, but in spite of growing interest in transdiagnostic approaches, group therapies have so far mostly been diagnosis specific. Aims: This study is aimed at evaluating a brief transdiagnostic cognitive behavioural group therapy (TCBGT) designed to treat both anxiety and depression among patients in primary care. Method: The participants were 287 adult patients in primary care with diagnoses of depression and/or anxiety disorders. They underwent a 5-week TCBGT. A mixed design ANOVA was used to evaluate differential effects of treatment according to diagnostic groups (anxiety versus depression) and number of diagnoses (co-morbidity). Results: Pre–post differences were significant and the treatment was equally effective for both anxiety disorders and depression. Number of diagnoses did not affect the outcome. Conclusions: The study indicates feasibility of the brief transdiagnostic group therapy for a wide range of mood and anxiety disorders in primary care. The results indicate that low intensity, brief transdiagnostic group therapies may be a feasible way to improve access to psychological therapies for a large number of patients.