Teacher job satisfaction is an essential factor for teachers' and school effectiveness and students' academic and educational achievement. The present research aims to identify variables that contribute to job satisfaction in a Portuguese sample of lower secondary education teachers, using data from the Teaching and Learning International Survey (TALIS) 2013. Two questionnaires were used to collect data: a school principal's questionnaire, and a teacher questionnaire. Hierarchical linear modelling was used to study the relation of school-level and teacher-level variables to job satisfaction. The results show that teacher-level variables are better predictors of teacher job satisfaction than school-level variables, except for the variable public/private school. In addition, variables related to interpersonal relations emerge as the most significant predictors of job satisfaction. The results suggest that, in order to improve, schools must take care of interpersonal relations, mainly at the classroom level, where most of the perceived job satisfaction seems to rest.