BACKGROUND: Sarcoidosis-associated fatigue is highly prevalent and is often reported as the most burdensome symptom of sarcoidosis. Management of fatigue is challenging, and evidence-based therapies are lacking. In this TIRED trial, we aimed to assess the effects of a 12-week online mindfulness-based cognitive therapy (eMBCT) on fatigue. METHODS: This study was a prospective, open-label, multicentre randomised controlled trial, conducted at three centres in the Netherlands. Eligible patients were 18 years or older, had stable sarcoidosis, and a score of more than 21 points on the Fatigue Assessment Scale (FAS). Patients were randomised into either the eMBCT or the control group. Participants completed patient-reported outcome measures at baseline, after intervention (T1), and 12 weeks after completion of eMBCT (T2). The primary outcome was the change in FAS score at T1 in the eMBCT group compared with the control group, assessed with the independent students't test in all patients who started the study. Secondary outcomes included within-group difference in FAS score at T1 and T2, between-group difference in FAS score at T2, and changes in the Hospital Anxiety and Depression Scale, the Freiburg Mindfulness Inventory-Short Form, and the Kings Sarcoidosis Questionnaire. The study was registered at the Netherlands Trial Register, NL7816. FINDINGS: Between June 5, 2019, and Oct 28, 2021, 99 patients were randomly assigned to the eMBCT (n=52) or the control (n=47) groups. Six patients withdrew consent after psychological screening before the start of eMBCT. Baseline FAS score was similar in both groups (34·57 [SD 6·07] for 46 patients in the eMBCT group and 35·51 [4·65] for 47 patients in the control group). Mean change in FAS score at T1 was -4·53 (SD 5·77; p<0·0001) in the eMBCT group and -1·28 (3·80; p=0·026) in the control group (between-group difference 3·26 [95% CI 1·18 to 5·33; p=0·0025]). Furthermore, the eMBCT group had a significant improvement in anxiety (mean between-group difference 1·69, 95% CI 0·22-3·16; p=0·025), depressive symptoms (1·52, 0·08-2·95; p=0·039), mindfulness (3·1, 0·70-5·49; p=0·022), and general health status (6·28, 2·51-10·06; p=0·002) at T1, compared with the control group. INTERPRETATION: 12 week eMBCT improves fatigue, anxiety, depression, mindfulness, and health status in patients with sarcoidosis-associated fatigue. FUNDING: Dutch Sarcoidosis Patient Association (Sarcoidose.nl). TRANSLATION: For the Dutch translation of the summary see Supplementary Materials section.