Aim: The aim of this prospective cohort study was to assess the effect of a pocket irrigator/evacuator device (IED) in the non-surgical treatment of peri-implantitis. Material and Methods: In total 24 patients having 38 implants diagnosed with peri-implantitis were included in this study. Peri-implant pockets were irrigated six times in three consecutive weeks. The primary outcome was bleeding on probing (BoP). Secondary outcome parameters included plaque index (Pl), suppuration on probing (SoP), probing pocket depth (PPD), marginal bone loss (MBL), presence and numbers of periodontal pathogens. Parameters were assessed at baseline and 3 months after the last treatment. Treatment pain perception was scored using the visual analog scale (VAS) after the first and last treatment. Results: At 3 months, IED treatment revealed significant reduction of peri-implant BoP (71% [±20] vs 57% [±28] [P =.014]) and peri-implant plaque scores (10 [±14] to 5 [±9] [P =.039] [T0 vs T3 respectively]). Significant reduction in mean peri-implant PPD from 4.92 mm (SD ± 1.28) to 4.66 mm (SD ± 1.35) (P =.041) was observed. In addition, a reduction in VAS pain score between the first and the last (6th) treatment was found (P =.039). No reduction in SoP (P =.088) was found. No changes in mean periodontal full mouth plaque, BOP, SOP and PPD levels, MBL and microbiological outcomes were found. Conclusion: Beneficial clinical effects in terms of BoP, PPD and PI were found at 3 months after IED treatment. However, the IED does not seem to effectively treat peri-implantitis in terms of disease resolution.