The study aimed to explore teachers' beliefs about student self-assessment (SSA) and the reasons for their use of it in their classrooms. A representative sample of 944 Spanish teachers (38.8% primary school sector, 54.0% secondary school sector and 7.2% university or adult education sector) were surveyed about SSA. Data were analysed using confirmatory factor analysis and structural equation modelling to determine statistically and theoretically significant predictors of teachers' self-reported use of SSA. Results showed that 90% of the teachers indicated having used SSA in their courses and 90% reported having positive experiences. A well-fitting structural model found five statistically significant predictors for the use of SSA: (1) positive experience with self-assessment, (2) belief in student participation in assessment, (3) willingness to include self-assessment as percentage of final grade, (4) self-assessment advantages and (5) having attended assessment courses. Statistically significant mean score differences for these factors were found according to level of employment and level of educational qualifications. In conclusion, teachers' values, attitudes and prior experiences with this type of assessment contributed a significant proportion of self-reported SSA usage.