Self-regulated learning (SRL) is crucial for successful lifelong learning and an important educational goal. For students to develop SRL skills, they need appropriate SRL support from teachers in the classroom. Teachers, who are aware of their students' strengths and weaknesses in SRL, can promote SRL more adaptively. This requires teachers to assess students' SRL skills accurately. However, there is little research on teachers' diagnostic competences in SRL. To address this research gap, the present exploratory study investigates teachers' content knowledge about SRL, assessment activities, and accuracy in judging their students' SRL. Furthermore, the study examines whether teachers' characteristics and competences in SRL are associated with the accuracy of their judgments. The study included 41 lower secondary school teachers and their 173 students. The students completed metacognitive knowledge tests on several SRL skills while the teachers made predictions about the students' metacognitive knowledge of those SRL skills. The results indicate that not all teachers were familiar with the assessment of SRL. Moreover, teachers exhibited greater familiarity with offline assessments of SRL than online assessments and a noteworthy proportion of teachers employed assessment activities that were not diagnostic of SRL. Low correlations between students' actual test scores and teachers' judgments generally revealed low accuracy for teachers in assessing their students' metacognitive knowledge of various SRL skills. Teachers' characteristics and competences in SRL were mainly uncorrelated with their judgment accuracy. Overall, these results highlight the need for further attention and support for teachers in developing their diagnostic competences in SRL.