Patient participation in treatment decision-making is one of the fundamental components of patient-centered care in psycho-oncology, which in recent years has evolved from an ethical principle related to patient autonomy to an important component in the quality of cancer care. In the biopsychosocial approach, the patient with cancer is not merely the recipient of medical services, but rather an individual with unique values, preferences, beliefs, life goals, and lived experience that should be taken into account in the treatment selection and evaluation process. Active patient participation in treatment decisions can facilitate psychological adjustment to the disease by increasing the sense of control, self-efficacy, agency, and trust in the treatment team. A cancer diagnosis is usually associated with a perceived threat to life, future, personal identity, independence, family relationships, and social roles, and this feature can lead to psychological distress, anxiety, depression, fear of relapse, hopelessness, and feelings of helplessness. In such a situation, the patient's participation in decision-making can compensate for some of the feeling of loss of control and help the patient play a more active role in the disease process. Shared decision-making makes sense when the patient receives information about treatment options, benefits, risks, possible complications, and consequences of each option in understandable language and has the opportunity to express their personal concerns, preferences, and priorities. This process can lead to greater alignment between treatment goals and the patient's individual values, thereby increasing satisfaction with care and health-related quality of life. Quality of life in cancer patients is a multidimensional concept and includes physical functioning, emotional state, social relationships, cognitive functioning, independence, family roles, and in some patients, spiritual and existential dimensions. When the patient has a real role in the treatment choice, they are more likely to align the outcomes of treatment with their life goals and not experience treatment as a mere set of medical measures. Participation can also improve the patient's psychological adjustment by increasing a sense of ownership over the treatment decision, reducing decision-making conflict, and fostering trust in the treatment team. In contrast, excluding the patient from the decision-making process may increase feelings of dependence, helplessness, and lack of control, and exacerbate psychological distress. However, optimal participation does not necessarily mean a complete assignment of decision-making responsibility to the patient, as some patients at different stages of the disease tend to delegate more responsibility to the physician or family due to the severity of symptoms, fatigue, anxiety, cognitive impairment, or personal preference. Therefore, the level of participation should be adjusted based on the psychological capacity, preferences, and needs of each patient. One of the important roles of a psycho-oncologist in this field is to identify psychological barriers to participation and to provide the conditions for the patient to be present in decision-making without feeling pressured or coerced. Assessment of psychological distress, anxiety, depression, fear of relapse, tolerance of ambiguity, and the level of self-efficacy can be an important prerequisite for effective decision-making. Patients who are in a state of severe distress may not process treatment information properly or have difficulty comparing treatment options, so psychological support should be placed alongside medical informing. On the other hand, the quality of doctor-patient communication plays an important role in transforming formal participation into real participation. Using understandable language, active listening, validating the patient's emotions, examining preferences, and providing opportunities for questioning can increase the sense of being seen and heard. Family presence is also important in many cancer patients, but family support is constructive when it does not replace the patient's agency and helps strengthen the patient's ability to express personal desires. From a research perspective, the relationship between participation and quality of life and distress should be investigated with regard to variables such as type of cancer, stage of disease, severity of treatment, socioeconomic status, health literacy, social support, and cultural characteristics, as these factors can change the intensity and direction of the relationship between participation and psychological outcomes. One of the important challenges is the time constraints of the treatment team and the complexity of medical information, which may reduce the opportunity for joint decision-making. Also, cultural differences in attitudes toward physician authority, family role, and the concept of autonomy can affect the desired level of participation. Therefore, the implementation of shared decision-making should be flexible, culturally sensitive, and based on the patient's real preferences. It is suggested that cancer centers consider patient participation as part of the patient-centered standard of care and develop communication therapy, joint decision-making, and distress assessment training for treatment team members. The use of clinical decision assistants, health literacy training, psycho-oncology counseling sessions, and regular quality of life assessment can increase the effectiveness of this process. Overall, patient participation in treatment decision-making can be considered as a bridge between medical care and the patient's psychological needs, which can provide the ground for improving quality of life and reducing psychological distress through strengthening agency, perceived control, trust, and alignment of treatment with individual values. From this perspective, the ultimate goal of psycho-oncology is not merely to reduce psychological symptoms, but to help the patient maintain dignity, authority, meaning, and quality of life throughout the course of facing cancer.