Purpose: The aim of this study was to investigate the retentive force of six different attachment systems used for mandibular single-implant overdentures, including two prototype large ball attachment designs. Materials and Methods: Two prototype ball attachments of larger dimensions (7.9 and 5.9 mm) and four commercially available ball and stud attachments of standard dimensions (2.25 and 4.0 mm) were evaluated on three identical test casts resembling an edentulous mandible with severe residual ridge resorption. Five samples from each attachment system (n = 30) were connected to three different implants (8.0-mm wide diameter, 3.75-mm regular diameter, and 4.0-mm regular diameter). An Instron testing machine with a computer software package was used to deliver a vertical dislodging force at a cross-head speed of 50 mm/min to each overdenture sample from the anterior direction. A total of 300 pull tests were conducted (50 per attachment system). The maximum load (retentive force) required to separate each overdenture from the supporting implant was then measured. Results: The highest retentive force (36.97 ± 2.23 N) was achieved with the 7.9-mm prototype ball attachment design, followed in a decreasing order by the 5.9-mm prototype ball attachment design (32.06 ± 2.59 N), the standard 2.25-mm ball attachment (17.32 ± 3.68 N), Locator white (12.39 ± 0.55 N), Locator pink (9.40 N ± 0.74 N), and Locator blue (3.83 ± 0.64 N). A statistically significant difference (P .0001) was found between all attachments. Conclusions: Attachment systems of larger dimensions provided higher retentive forces for mandibular single-implant overdentures. Further in vitro and in vivo research is necessary to determine prosthodontic outcomes with these attachments in edentulous patients prior to their routine clinical use internationally.