Self Determination Theory (SDT) is an important theoretical framework used in psychological counseling to increase intrinsic motivation, strengthen psychological well-being and support behavioural changes. This study examines the applications and effects of SDT in psychological counseling in detail. Deci and Ryan SDT focuses on the needs for autonomy, competence and relatedness to increase the intrinsic motivation of individuals. Meeting these basic needs has positive effects on the psychological well-being of clients. SDT applications in psychological counseling stand out with their effectiveness in increasing client engagement and commitment to therapy. The autonomy-supportive approaches of SDT encourage clients to participate more actively in the therapy processes, which positively affects therapeutic outcomes. Research shows that SDT increases clients' intrinsic motivation and this motivation leads to high success rates in therapeutic processes. In the study, the application examples and case studies section shows how SDT is applied in various therapeutic contexts and what results it yields. The case studies illustrate how SDT supports clients' psychological well-being and is effective in behavioural change processes. Clinical research and meta-analyses provide important findings supporting the general validity and effectiveness of SDT. In conclusion, SDT is an effective approach in psychological counseling to increase clients' intrinsic motivation, strengthen their psychological well-being and support long-term changes.