Contributing to the knowledge about teachers’ educational use of social media, the aim of this article is to explore Swedish secondary school teachers’ understanding of their duties of care and responsibilities for what pupils do on social media. The article draws on data from a mixed method study, consisting of an interview study and a national survey. The findings from the survey show that some groups of teachers are more likely than others to express responsibility for pupils, especially teachers who use many different social media, those who use them in contact with pupils, teachers working at private schools, and female teachers. The subjective experiences from the interviews are supported by the findings from the survey – indicating a correlation between social media usage and level of perceived responsibility. The more the teachers use social media, and thereby get access to more personal information about the pupils, the greater the likelihood that the teachers will perceive that they have some responsibility for what the pupils do on social media. The meaning of the findings is discussed in relation to the theory of caring ethics.