Purpose: Transient ocular hypotony commonly occurs after 23-gauge (23G) vitrectomy. To assess possible causes, we visualized the sclerotomy site and pars plana by anterior segment optical coherence tomography (AS-OCT). Procedures: We prospectively analyzed the intraocular pressure (IOP) and findings related to clinical hypotony, assessed by AS-OCT, in 13 consecutive eyes of 13 patients receiving 23G vitrectomy. Five patients receiving a 20G vitrectomy served as controls. Results: The mean IOP after 23G vitrectomy was 11 mm Hg (range: 5–32 mm Hg; 13 eyes) at postoperative day 1. In contrast to controls, AS-OCT showed a scleral gap at the incision site in 22/39 sclerotomies (10/13 eyes), and subclinical choroidal detachment in 9/13 eyes. Conclusions: A scleral gap and choroidal detachment at the incision site are frequent findings following 23G vitrectomy. These subclinical features may account for postoperative hypotony as they are absent in 20G vitrectomy and scleral sutures. Hypotony following 23G vitrectomy is a self-limiting phenomenon and usually does not require therapeutic intervention.