The purpose of this Ethnonursing study was to describe characteristics of a mentally healthy person and to identify protective factors associated with positive mental health from Thai adolescents' perspectives. Three orienting questions were addressed: (1) What do Thai adolescents define as characteristics of being a mentally healthy person? (2) What do Thai adolescents do to maintain their mental health? and (3) What factors do Thai adolescents believe facilitate their mental health? Data consisted of transcriptions from individual and group interviews and participant observations of 23 high school students from a co-educational school in southern Thailand, during a three and a half month period. Data were analyzed using Leininger's Phases of Ethnonursing Analysis for Qualitative Data. Ethical issues and the rigor were considered in the study. Emerging themes were validated by informants, an expert in qualitative approaches and two experts in psychiatric/mental health. The findings were translated into English and validated by a bilingual Thai professor. Characteristics of a mentally healthy person were identified as (1) having a good mood: smiling, being cheerful and polite, and being worry-free, (2) positive thinking: a good attitude towards one's self and focusing on the positive aspects of others and of situations, and (3) good social relationships: being friends with others and having the ability to manage problems. Five methods the Thai adolescents used to maintain mental health included expressing feelings through actions and in words, distracting themselves from problems, seeking help, using a cognitive strategy, and religious practices. Eight protective factors for mental health included peers, family, school, mass media, religion, physical environment, economics, and physical health. The findings showed that traditional Thai values of social network and religion strongly influenced informants' ideas even though Thailand is becoming more westernized and it is experiencing a new industrialized period. Implications for culturally congruent nursing interventions in clinical practice, education, research, and recommendations for future studies are presented.