Objective To compare cataract surgery with implantation of a Schlemm canal microstent with cataract surgery alone for the reduction of intraocular pressure (IOP) and medication use after 24 months. Design Prospective, multicenter, single-masked, randomized controlled trial. Participants Subjects with concomitant primary open-angle glaucoma (POAG), visually significant cataract, and washed-out modified diurnal IOP (MDIOP) between 22 and 34 mmHg. Methods Subjects were randomized 2:1 to receive a single Hydrus Microstent (Ivantis, Inc, Irvine, CA) in the Schlemm canal or no stent after uncomplicated phacoemulsification. Comprehensive eye examinations were conducted 1 day, 1 week, and 1, 3, 6, 12, 18, and 24 months postoperatively. Medication washout and MDIOP measurement were repeated at 12 and 24 months. Main Outcome Measures The primary and secondary effectiveness end points were the proportion of subjects demonstrating a 20% or greater reduction in unmedicated MDIOP and change in mean MDIOP from baseline at 24 months, respectively. Hypotensive medication use was tracked throughout the course of follow-up. Safety measures included the frequency of surgical complications and adverse events. Results A total of 369 eyes were randomized after phacoemulsification to Hydrus Microstent (HMS) and 187 to no microstent (NMS). At 24 months, unmedicated MDIOP was reduced by ≥20% in 77.3% of HMS group eyes and in 57.8% of NMS group eyes (difference = 19.5%, 95% confidence interval [CI] 11.2%–27.8%, P Conclusions This 24-month multicenter randomized controlled trial demonstrated superior reduction in MDIOP and medication use among subjects with mild-to-moderate POAG who received a Schlemm canal microstent combined with phacoemulsification compared with phacoemulsification alone.