Aim: Oral hygiene-related self-efficacy (OHSE) describes one's confidence to successfully execute oral hygiene behaviour. The aim of this study was to investigate the long-term course of OHSE in patients during initial periodontal therapy (IPT) and supportive periodontal therapy (SPT) and its association with clinical parameters. Materials and Methods: Patients diagnosed with periodontitis, undergoing either IPT or SPT, were evaluated at two timepoints. Clinical examination included pocket probing depths (PPDs), clinical attachment loss (CAL), bleeding on probing (BOP), plaque index (PI) and gingival index (GI). Patients' OHSE was assessed with a questionnaire. Statistical analyses included t-tests and linear regression models. Results: Ninety-eight patients from an initial group of 201 patients were evaluated after 4 years. The overall OHSE score increased significantly in the IPT group (mean 11.65 ± 15.6, p =.001). The increase in the OHSE category ‘interdental cleaning’ was significantly correlated with a decrease in the number of pockets requiring treatment (Spearman correlation r_s = −.2349, p =.022) and periodontal inflamed surface area (PISA) (r_s = −.2099, p =.042). Conclusions: Patients under IPT showed a significant increase of OHSE compared to those under SPT. Improved OHSE, particularly in interdental cleaning, appears to be associated with sustained success of periodontal therapy.