In-laboratory polysomnography is recommended for the evaluation of obstructive sleep apnea (OSA) in youth with Down syndrome. However, insufficient sleep laboratories are available, particularly for youth with neurocognitive disabilities such as Down syndrome. We hypothesized that level II home sleep apnea testing (HSAT) would be feasible, acceptable, and accurate in detecting polysomnography-defined moderate-severe OSA in youth with Down syndrome. Youth 6 to 25 years old with Down syndrome were recruited to undergo in-home level II HSAT with electroencephalogram and in-lab polysomnography. Parents completed questionnaires assessing feasibility, acceptability, and test preference. HSAT, scored blinded to polysomnography result, were compared to reference polysomnography. Forty-three youth (23 female) aged [median (range)] 15.5 (6.1, 25.1) years participated in the study. Forty-one participants were able to complete HSAT and 41 completed polysomnography, with 40 who underwent both tests. HSAT was preferred to polysomnography by 73.7% of parents. Total sleep time for HSAT was 437 ± 123 minutes vs 366 ± 90 minutes for polysomnography (P = .003). Obstructive apnea-hypopnea index by polysomnography was 12.7 events/h (0.2, 113.8), and 32 youth (80%) who completed all testing had OSA. Compared to polysomnography, sensitivity of HSAT was: 0.81, specificity was 0.75, accuracy was 0.8 including 2 youth whose HSAT demonstrated OSA when polysomnography did not. In youth with Down syndrome, level II HSAT was well-tolerated, preferred compared to in-lab polysomnography, and had good accuracy for detecting moderate-severe OSA. Level II HSAT could provide a means for expanding the evaluation of OSA in youth with Down syndrome. Cielo CM, Kelly A, Xanthopoulos M, etal. Feasibility and performance of home sleep apnea testing in youth with Down syndrome J Clin Sleep Med. 2023;19(9):1605-1613.