Teachers’ formative feedback provides motivational and cognitive support for student engagement in learning and raises their achievement. Because feedback arises from assessment, teacher conceptions of both assessment and feedback matter to how feedback is practised. This study determines how teacher conceptions of assessment and feedback are linked to each other and to self-reported practices in a repeated measures survey 18 months apart. All factors in the Swedish Teachers Conceptions of Assessment inventory predicted all six Conceptions of Feedback, which fully mediated influence of assessment conceptions on teachers’ self-reported formative feedback practices. Two conceptions of feedback (i.e., feedback improves learning, and students ignore feedback) were important predictors of teachers’ practice, and these feedback conceptions were predicted by the corresponding conceptions of assessment (i.e., it improves learning and it is ignored by students). The results imply that teacher formative feedback practices depend on an improvement-oriented conception of assessment as a prior belief.