--- - |2+ Abstract Background Attachment insecurity is associated with poor mental health across the life span, and increasing evidence suggests that emotion regulation difficulties are implicated in these associations. To date, research has focused largely on adult attachment in the context of romantic relationships; however, close friendships are well‐established predictors of mental health and well‐being, and little is known about the influence of attachment in the context of close friendships on mental health, and the potential role of emotion regulation. The present research examined whether attachment to friends was associated with mental health and interpersonal functioning, and whether emotion regulation mediated these associations. Method A sample of 243 adults (Mage = 20.91 years) completed an online questionnaire assessing attachment to friends, emotion regulation, mental health, and interpersonal functioning. Results Emotion dysregulation mediated the association between attachment anxiety and depression, anxiety, stress, and interpersonal distress. There were also direct effects of attachment anxiety on symptoms of anxiety and interpersonal distress. Emotion dysregulation did not mediate the association between attachment avoidance and psychosocial outcomes, though attachment avoidance directly predicted depression and interpersonal distress. Conclusions This research highlights that emotion dysregulation may be one mechanism underlying the association between anxious attachment to friends and poor psychosocial functioning. Clinical interventions may therefore benefit from assessing attachment in the context of friendships, and targeting emotion dysregulation for those high in attachment anxiety. - Clinical Psychologist, Volume 22, Issue 2, Page 158-167, July 2018.