School-based universal well-being screening is proposed within the Multi-tiered Systems of Support approach to collect data on school and individual well-being. Universal screening allows for data-based informed decision, allocating each pupil or set of pupils in universal, selective and/or indicated structured interventions and supports. However, schools require adequate and validated measures of both positive well-being and indicators of psychological health problems. This study focused on the adaptation and validation of the Student Subjective Wellbeing Questionnaire (SSWQ) in Portugal, a tool directed to positive well-being screening. After a translation process, data was collected in 10 schools. A sample of 942 school pupils in the 3rd cycle was randomly split into two subsamples. First was conducted on one subsample, and then, CFA was conducted in the other. Gender and school year invariance were also conducted with the CFA subsample. Internal consistency was likewise analysed. The analysis finished with convergent and discriminant validity. Overall, results supported a second-order factor model (subjective well-being), including four first-order factors, with the elimination of one item and changing one item to another subscale. Results showed good reliability of each factor (all above 0.70). Full measurement invariance was reached for gender (male and female) and school year (7th, 8th and 9th grades). Convergent validity was found with measures of satisfaction with school, group identification with peers and school social support, and divergent validity was found with BPM-Y. Altogether, results support that SSWQ-PG can be used to screen for subjective well-being in schools, helping schools in identifying and addressing pupils’ mental health needs more effectively.