Purpose: This in vitro study aimed to evaluate and compare the effect of different bar designs on the retention forces of implant-retained maxillary overdentures. Materials and Methods: A maxillary edentulous acrylic resin model without alveolar undercut was fabricated. Four implant analogs were placed in the canine and second premolar regions. Overdentures were made and attached to the analogs with Dolder bar, Hader bar, and milled bar attachments. A universal testing machine was used to measure axial (resistance to vertical displacement) and nonaxial (resistance to anterior, posterior, and lateral displacement) retention forces (in Newtons). Measurements were made at the start of the experiment (initial retention) and after 540 cycles of denture insertion and removal to simulate 6 months of clinical function (final retention). Results: Hader bar had the highest retention after insertions and removals, and Dolder bar had the lowest retention. Vertical displacement showed the highest retention for Dolder and milled bars, and posterior displacement showed the highest retention for Hader bar. Lateral displacement recorded the lowest retention for all bar designs. No significant difference in axial retention loss was observed between different bar designs. Dolder bar showed the highest nonaxial retention loss, and milled bar showed the lowest retention loss. Conclusion: Hader bar is recommended to retain maxillary implant overdentures, as it was associated with higher axial and nonaxial retention compared with Dolder and milled bars after 6 months of simulated denture wear. Keywords: Hader bar, implant, maxillary, milled bar, overdenture, retention