This study investigates the impact of nonsurgical periodontal treatment (NSPT) on oral health-related quality of life (OHRQoL) in patients with periodontitis stages (S)2 and S3, and the factors associated with the prediction of patient-reported outcomes. Periodontitis patients (n = 68) with moderately deep periodontal pockets were recruited. Responses to the Oral Health Impact Profile (OHIP)-14 questionnaire and clinical parameters including plaque index, bleeding on probing (BOP), probing pocket depth (PPD), and clinical attachment loss (CAL) were recorded. All patients received supra- and subgingival professional mechanical plaque removal. All clinical parameters and questionnaire responses were recorded again 3 months after NSPT. Clinical parameters and OHIP-14 scores for both stages of periodontitis were significantly improved 3 months after treatment. However, participants with periodontitis S3 had significantly higher total OHIP-14, physical pain, and functional limitation domains scores than periodontitis S2 cases. Baseline CAL, BOP, and the presence of PPD in anterior teeth were positively associated with increased OHIP-14 scores after NSPT. NSPT improved OHRQoL in participants with periodontitis S2 and S3. This was more pronounced in participants having periodontitis S3 than S2. Poorer OHRQoL could be anticipated in people having severe CAL, high BOP, and presence of pockets in the anterior teeth.