Background: This study investigated the effect of a 4-week free-sugar avoidance on periodontal parameters during periodontal therapy. Methods: Twenty-one patients with untreated periodontitis and daily free-sugar intake were allocated to a sugar avoidance group (SAG) and a control group (CG). The SAG received a 45-min dietary consultation and was instructed to avoid free sugars during the following 4 weeks after subgingival instrumentation, while the CG continued with their regular diet. Bleeding on probing (BOP), plaque control record, body weight (BW), visceral fat (FATv), and a food frequency questionnaire (FFQ) were collected at baseline (T1), 4 weeks (T2), and 8 weeks (T3) after subgingival instrumentation. Results: The main outcome parameter BOP was significantly reduced at T2 by 40.3% ± 15.54 in the SAG and 34% ± 12.47 in the CG (intra-p value both <0.001, inter-p value 0.361). A linear regression analysis of changes at patient level adjusted for age and FATv revealed a significant group difference for BOP (regression coefficient = –6.8; p = 0.019). Significant reductions were observed in BW, FATv and mean daily intake of free sugars (–14.4 g/day), and a significant increase of vitamin C derived from fruits (75.89 mg/day) at T2 in the SAG only. Conclusion: This study may indicate additional beneficial effects of a sugar avoidance on periodontal and metabolic parameters, and nutritional intake during periodontal therapy. German Clinical Trials Register (DRKS00026699). Plain Language Summary: The current widespread free-sugar consumption is linked to an increasing incidence of chronic non-communicable diseases. Data indicate a relationship between sugar intake and a higher prevalence of periodontitis and increased gingival inflammation. This study showed that free-sugar avoidance after periodontal therapy had additional beneficial effects on periodontal and metabolic parameters in 10 test and 11 control patients. After 4 weeks of avoiding free sugars like sweets, processed white flour, juice, and so forth, periodontal bleeding was significantly reduced in both groups (–40.3% test group, –34% control group). Further regression analysis revealed a significant difference between groups favoring the intervention. Additionally, body weight and visceral fat were significantly reduced in the intervention group, only. To avoid sugar, patients were allowed to replace it with whole fruit, which led to increased levels of micronutrients such as vitamin C. Therefore, free-sugar avoidance may be of therapeutic benefit in addition to periodontal therapy. Further research is needed to investigate this effect in larger cohorts.