This case series demonstrated the regeneration of peri-implant keratinized mucosa (KM) in the posterior mandibles by a modified apically positioned flap and xenogeneic collagen matrix (CM). This modified surgical approach includes a midcrestal incision in the residual KM, partial-thickness flap reflection and apical positioning, removal of submucosal tissue, and CM adaptation. Six patients with 18 implants were recruited. The mean regenerated KM width was 4.81 ± 0.69 mm after 3 months of healing. Histologic analysis of the regenerated KM revealed similar architecture to the native KM.