Background and Aim: Parental involvement in the mental health care of hospitalized children entails active, continuous, and purposeful engagement throughout the hospital stay. By participating in care activities physical, emotional, and social parents enhance their child's psychological well-being, reduce stress, and improve adaptation and treatment outcomes. As a core component of family centered care, understanding the nature, extent, and determinants of this involvement is essential. This study systematically reviews the existing evidence on parental participation in the mental health care of hospitalized children. Methods: This systematic review was conducted in accordance with PRISMA ۲۰۲۰ guidelines. A comprehensive literature search was performed in PubMed, Scopus, ScienceDirect, Google Scholar, and SID databases using the English keywords (Parental Involvement, Parental Participation, Mental Health, Hospitalized Children) and their Persian equivalents, covering the period from January ۲۰۱۵ to January ۲۰۲۵. Articles were screened based on predefined inclusion criteria-presence of keywords in the title or abstract, full text availability, and focus on the target population (parents of hospitalized children). After duplicate removal, ۱۸ studies (۱۲ in English, ۶ in Persian) were included for qualitative synthesis. Results: Content analysis revealed that in most studies, parental involvement in the mental health care of hospitalized children was at a moderate level, with variations shaped by cultural background, health system structure, and hospital setting. Participation patterns included active engagement in daily care, continuous emotional support, attendance in psychotherapy or counseling, collaboration in clinical decision making, and involvement in rehabilitative or play therapy activities. Determinants clustered into three domains: ۱. Parental factors knowledge and awareness, attitudes, emotional and psychological status, communication skills, socioeconomic status. ۲. Child/illness factors severity and type of mental health condition, need for specialized or prolonged care, child's interaction ability. ۳. Organizational/environmental factors hospital policies, team support, visiting flexibility, facility quality. Structured team support, targeted parental education, and respectful communication enhanced participation, while insufficient training, time constraints, emotional burden, and inconsistent policies reduced it. Conclusion: Parental involvement benefits hospitalized children's mental health by reducing anxiety, improving adaptation, and strengthening parent-child interaction, yet current engagement levels remain below optimal. Maximizing these benefits requires coordinated strategies beginning at admission and extending post discharge, supported by evidence based education, ongoing support, and strong communication with healthcare teams. Future qualitative and interventional studies should further inform policies to enhance parental participation.