Arachnoid cysts are the most common incidental findings on brain scans and are considered benign congenital structures. However, recent research associates cysts with affective- and cognitive symptoms. Despite this, the prevailing approach is that only signs of mass effect or nerve compression are taken as indications for neurosurgical treatment. On a brain MRI of a 32-year-old woman with depression, collected as a part of the multi-modal deep-phenotyping research project, the BrainDrugs-Depression study, we found a 38 × 17 mm arachnoid cyst located on the frontoparietal convexity just above the right frontal lobe. As part of the project, the study participants also underwent high-density EEG as well as hot- and cold cognitive testing. The cyst did not cause structural or functional abnormalities in the brain as evaluated with MRI, EEG, and cognitive tests. It was concluded to be unrelated to her current depressive episode. Although we found no association between the cyst and the woman's depressive state, this case highlights the importance of excluding organic etiologies when seeing patients with depression.