Background: Considering the importance of psychological interventions for adults who stutter (AWS), the present study compared the effectiveness of Cognitive Behavioral Group Therapy (CBGT) and Mindfulness and Acceptance Group Therapy (MAGT), in combination with Speech Therapy (ST) in AWS. Materials and Methods: A randomized clinical trial was conducted with 36 AWS in Tehran and Alborz provinces in Iran from September-2019 to September-2020. Participants were assigned to CBGT + ST and MAGT + ST groups. Outcomes variables, the fear of negative evaluation scale (FNE) and the social avoidance and distress scale (SAD), were measured at four stages: (1) pretest, (2) after MAGT/CBGT, (3) after ST, and (4) at 6-month follow-up. Results: The results of repetitive measure-analysis on variance showed that CBGT + ST and MAGT + ST were significantly effective (P = 0.001 and P = 0.003) on SAD. The same analysis showed that CBGT + ST was significantly effective (P = 0.001) on FNE. The results of Analysis of Covariance showed that there was no significant difference between CBGT + ST and MAGT + ST on SAD but CBGT + ST was more effective than MAGT + ST at stage 3 (P < 0.05) on FNE. Conclusion: Data indicated that CBGT + ST and MAGT + ST were both effective for SAD of AWS. Regarding FNE, unlike MAGT + ST, CBGT was effective either alone or in combination with ST for AWS. Further studies are needed to confirm the results of this study.