Hydraulic sinus lift and augmentation may be successfully performed using a dedicated implant device designed according to residual bone height (RBH). The aim of this study was to evaluate whether a flapless surgical approach might negatively influence the outcome. A total of 40 consecutive patients (16 men and 24 women) were included in the study, 20 in each group (minimal flap/control versus flapless/study). Inclusion criteria were ≥ 3 mm RBH, ≥ 6 mm width of the residual alveolar ridge according to preoperative cone beam computerized tomography, and ≥ 8 mm buccopalatal keratinized gingiva for the flapless group. Primary outcome parameters included intraoperative membrane perforation. Secondary outcome parameters included postoperative infection, soft tissue healing, bone gain, and short-term dental implant survival. Mean RBH was 4.6 mm. No intraoperative membrane perforations and no postoperative infections were observed. Mean bone gain height was similar for both groups, at 11 mm. Soft tissue healing was observed within 2 months. In all cases, second-stage surgery allowed generation of at least 2 mm of keratinized gingiva buccally. All implants were osseointegrated at second-stage surgery. The use of a flapless approach to maxillary sinus augmentation using an implant device based on RBH yields predictable results.