BackgroundHIV is characterised by high rates of comorbidity with mental health conditions including depression, as such, the detection and treatment of comorbid depression is critical to achieve viral load suppression. This study evaluated the effectiveness of a collaborative care intervention for depression among adults with comorbid depression symptoms receiving ART in primary health care (PHC) facilities. MethodsWe conducted a pragmatic cluster-randomised trial in 40 clinics in the North West province of South Africa. PHC clinics were stratified by sub-district and randomised in a 1:1 ratio. Participants were ≥ 18 years, receiving ART, and had depression symptoms indicated by Patient Health Questionnaire-9 (PHQ-9) score ≥ 9. Intervention clinics received: i) supplementary mental health training and clinical communication skills for PHC nurses; ii) workshops for PHC doctors on treating depression; and iii) lay counselling services. Using mixed effects regression models, we assessed co-primary outcomes of PHQ-9 response at 6 months (≥50 % reduction in baseline PHQ-9 score) and viral load suppression at 12 months (viral load<1000 copies/mL). ResultsThe intervention had no effect in PHQ-9 response (49 % vs 57 %, risk difference (RD) = −0.08, 95 % CI = -0.19; 0.03, p = 0.184) or viral load suppression (85 % vs 84 %, RD = 0.02, 95 % CI = −0.01; 0.04, p = 0.125). Nurses referred 4298 clinic patients to counsellors, however, only 66/1008 (7 %) of intervention arm participants were referred to counsellors at any point during the study. LimitationsThe highly pragmatic approach of this trial limited exposure to the counselling component of the intervention and referral to doctors for initiation of antidepressant treatment was extremely low. ConclusionThe trial showed no effect of a district-based intervention to strengthen collaborative care for depression. The trial revealed the extent of the treatment gap in the context of scaling up mental health services. Trial registrationClinicalTrials.gov (NCT02407691); Pan African Clinical Trials Registry (201504001078347).