Researchers have identified general pedagogical knowledge (GPK) as a relevant category of teacher knowledge. However, hardly any study has examined the relationship between GPK of teachers and the instructional quality delivered to their students. This article therefore investigates the relationship between teachers' GPK assessed via a standardised paper-pencil test and the quality of their instruction rated by their students. A sample of 246 in-service teachers at vocational schools in Austria is used. Teachers' GPK positively correlates with students' perceptions of effective classroom management, generic teaching methods/teacher clarity and teacher-student relationships. Regression analysis shows that GPK is a significant predictor for instructional quality even when controlled for teacher education grades, teacher personality (Big-Five) and teaching experience. Implications of teachers' GPK as a resource for their teaching, limitations of the study and perspectives for future research are discussed.