Evidence of the negative impact of depression on glycaemic control is equivocal, and diabetes-related distress has been proposed as potential mediator. 466 diabetes patients were cross-sectionally assessed for depression (Center for Epidemiologic Studies Depression Scale), diabetes-related distress (Diabetes Distress Scale), and glycaemic control (HbA1c). We distinguished the associations of depression and diabetes distress with glycaemic control using analysis of variance and multiple regression. Neither patients with depression only nor diabetes distress only differed significantly from controls regarding HbA1c. However, HbA1c was substantially increased when both conditions were present (9.2 vs. 8.6 %, P = 0.01). As in previous studies, we observed a significant association between depression and hyperglycaemia (P < 0.01). However, a mediation analysis revealed that this association in fact depended on the presence of diabetes distress (P < 0.01). Depression seems to be associated with hyperglycaemia particularly when accompanied by diabetes distress, suggesting that adjusting clinical procedures regarding diabetes distress may facilitate the identification and care of high-risk patients.