Introduction Retrospective studies conducted in psychiatric inpatient wards have shown a relation between the intensity of daylight in patient rooms and the length of stay, pointing to an antidepressant effect of ambient lighting conditions. Light therapy has shown a promising antidepressant effect when administered from a light box. The emergence of light-emitting diode (LED) technology has made it possible to build luminaires into rooms and to dynamically mimic the spectral and temporal distribution of daylight. The objective of this study is to investigate the antidepressant efficacy of a newly developed dynamic LED-lighting system installed in an inpatient ward. Methods and analysis In all, 150 inpatients with a major depressive episode, as part of either a major depressive disorder or as part of a bipolar disorder, will be included. The design is a two-arm 1:1 randomised study with a dynamic LED-lighting arm and a static LED-lighting arm, both as add-on to usual treatment in an inpatient psychiatric ward. The primary outcome is the baseline adjusted score on the 6-item Hamilton Depression Rating Scale at week 3. The secondary outcomes are the mean score on the Suicidal Ideation Attributes Scale at week 3, the mean score on the 17-item Hamilton Depression Rating Scale at week 3 and the mean score on the World Health Organisation Quality of Life-BREF (WHOQOL-BREF) at week 3. The spectral distribution of daylight and LED-light, with a specific focus on light mediated through the intrinsically photosensitive retinal ganglion cells, will be measured. Use of light luminaires will be logged. Assessors of Hamilton Depression Rating Scale scores and data analysts will be blinded for treatment allocation. The study was initiated in May 2019 and will end in December 2021. Ethics and dissemination No ethical issues are expected. Results will be published in peer-reviewed journals, disseminated electronically and in print and presented at symposia. Trial registration number NCT03821506; Pre-results.