Despite the widespread prevalence, teacher moonlighting is one of the under-researched areas. Using data from 313 public primary school teachers in Ilala District, this study examined the determinants of teacher moonlighting. The findings show that 39.4% of teachers had a secondary income generating activity. Sex and age of the teacher were significant predictors of the decision to moonlight. Further, the study findings show that the older the teacher is, the more likely the teacher is to moonlight. The results confirm the proposition that moonlighting in Tanzania is used by formal sector workers as a transition into self-employment after retirement.