Background In chronic central serous chorioretinopathy, fluid accumulates in the subretinal space and causes permanent vision loss in ≈ 30% of patients. There is no definitive treatment. Previous research suggests that the mineralocorticoid receptor antagonist eplerenone is effective but it is not licensed for chronic central serous chorioretinopathy. Objectives The objective was to evaluate whether or not eplerenone was safe and superior to placebo for treating chronic central serous chorioretinopathy. We also aimed to set up a biobank of DNA, serum and plasma samples from treatment-naive participants for future research. Design The trial was a parallel, randomised (1 : 1 ratio), multicentre, double-masked, placebo-controlled superiority trial comparing eplerenone plus usual care with placebo plus usual care. Participants were randomly allocated to eplerenone or placebo using a secure online system that returned a unique number corresponding to a bottle of the investigational medicinal product. Participants, clinical care teams, pharmacists, outcome assessors and the trial management group were masked. Setting The trial took place in 22 NHS hospitals in the UK. Participants Eligible participants were patients aged 18–60 years with treatment-naive chronic central serous chorioretinopathy of at least 4 months’ duration, a best corrected visual acuity score of 54–85 letters and no other conditions affecting visual acuity or contraindications to taking eplerenone or placebo. Interventions The intervention was oral eplerenone (25 mg/day for 1 week, increased to 50 mg/day for up to 12 months). Placebo was a lactose-filled capsule that appeared identical to the overencapsulated eplerenone tablets. To maintain blinding, participants in the placebo group followed the same dose escalation schedule as the eplerenone group. Usual care was included in both groups and was administered at the discretion of clinicians. Main outcome measures The primary outcome was best corrected visual acuity score at 12 months. Secondary outcomes were low-luminance visual acuity, central subfield retinal thickness, change in subretinal fluid thickness, systemic and ocular adverse events, macular atrophy of the retinal pigment epithelium, subfoveal choroidal thickness, choroidal permeability, resolution of subretinal fluid, time to recurrence of subretinal fluid, fundus fluorescein angiography phenotype, incidence of chronic central serous chorioretinopathy in the fellow eye, and patient-reported visual function. Results Between 11 January 2017 and 22 February 2018, 57 participants were randomised to eplerenone and 57 to placebo; 57 and 54 participants, respectively, were included in the analysis of the primary outcome. The modelled mean best corrected visual acuity score at 12 months in the eplerenone and placebo groups was 80.4 letters (standard deviation 4.6 letters) and 79.5 letters (standard deviation 4.5 letters), with an estimated difference between groups of 1.73 letters (95% confidence interval –1.12 to 4.57 letters; p = 0.24). Hyperkalaemia occurred in eight participants in each group (14%). No serious adverse events occurred in the eplerenone group; three unrelated serious adverse events occurred in the placebo group. Limitations Limitations included the inability to prevent co-treatments and discontinuation of the investigational medicinal product in the event of resolution or hyperkalaemia. Conclusions Eplerenone was safe but not superior to placebo in improving best corrected visual acuity in people with chronic central serous chorioretinopathy during 12 months of follow-up. In future work, ophthalmologists should investigate alternative treatments for this condition, which remains complicated to treat. Trial registration Current Controlled Trials ISRCTN92746680. Funding This project was funded by the Efficacy and Mechanism Evaluation (EME) Programme, a MRC and National Institute for Health Research (NIHR) partnership. This will be published in full in Efficacy and Mechanism Evaluation; Vol. 8, No. 2. See the NIHR Journals Library website for further project information.