Numerous types of mandibular advancement devices (MADs) are available to treat patients with obstructive sleep apnea, varying from noncustom to custom devices. Only a limited number of studies have been performed to determine whether a noncustom MAD could be used to predict treatment success of a custom MAD. In this study, we investigated the potential of a new-generation noncustom MAD, by comparing its effectiveness with a custom MAD. We hypothesized that the effectiveness of the devices is similar with regard to both objective (polysomnography) and self-reported (questionnaires, adherence, and patient satisfaction) outcomes. This was a single-center prospective randomized crossover study including a consecutive series of patients with obstructive sleep apnea. Patients were randomized to start either with the noncustom or custom MAD. Both MADs were applied for 12 weeks, followed by polysomnography with MAD in situ and questionnaires. After the first 12 weeks of follow-up, a washout period of 1 week was applied. Equal effectiveness was defined as no significant differences in both objective and self-reported outcomes between both devices. Fifty-eight patients were included; 40 completed the full follow-up. The median apnea-hypopnea index significantly decreased from 16.3 (7.7, 24.8) events/h to 10.7 (5.6, 16.6) events/h with the custom MAD (P = .010) and to 7.8 (2.9, 16.1) events/h with the noncustom MAD (P < .001). Self-reported outcomes significantly improved in both groups. No significant differences were found between both devices. The effectiveness of a noncustom and custom MAD is comparable, which suggests that a noncustom MAD can be used as a selection tool for MAD treatment eligibility to improve MAD treatment outcome. Registry: Netherlands Trial Register; Name: The Use of a Boil and Bite Mandibular Advancement Device vs a Custom Mandibular Advancement Device in Obstructive Sleep Apnea Management; URL: https://www.trialregister.nl/trial/7249; Identifier: NL64738.100.18. Bosschieter PFN, Uniken Venema JAM, Vonk PE, et al. Equal effect of a noncustom vs a custom mandibular advancement device in treatment of obstructive sleep apnea. J Clin Sleep Med. 2022;18(9):2155-2165.