Promoting mental health literacy among adolescents is crucial for early prevention and intervention of mental health problems in school settings. This study aimed to develop and validate the Spanish Mental Health Support Scale for Adolescents (MHSSA). This study involved 578 responses (ages 13–52 years, M = 20.28, SD = 6.29) from both secondary school students in Burgos and future school health education professionals in Burgos and Oviedo, Spain. The questionnaire was composed of the standardised Spanish MHSSA scale and six additional standardised scales. Exploratory and confirmatory factor analyses supported a two-factor structure (α = 0.88), comprising helpful and harmful intentions subscales. The Spanish MHSSA also demonstrated good test–retest reliability (ICC = 0.88). Initially, adolescents exhibited lower levels of mental health support behaviours compared to future school health educators, while post-intervention results showed significant improvements in support-seeking behaviours. Higher-quality support intentions were observed among women, post-test participants, individuals with prior lived experience of mental health problems, and future health professionals. These results demonstrate reasonable construct validity, further supported by the finding that scores on the Spanish MHSSA are also associated with lower social restrictive attitudes toward mental health problems (measured by the CAMI) and higher mental health literacy (measured by the MHLI). The findings indicate that the Spanish MHSSA is a valid and reliable tool for assessing both adolescents' and school health educators' intentions to engage in mental health–supportive behaviours.
