The present study examined teacher sensitivity as a potential moderator of the relationship between children's temperament in infancy and their social development in first grade. Using data from the Study of Early Child Care and Youth Development (NICHD SECCYD), we found that first grade teachers who were sensitive and responsive to students' individual needs tended to have a buffering effect against poor peer interactions during unstructured play for children with difficult temperament. Furthermore, for children low in temperamental approach, greater teacher sensitivity was linked to more positive peer interactions. Greater teacher sensitivity was also related to fewer negative peer interactions for children rated high in negative affect and low in adaptability. Our findings suggest teacher sensitivity is an important factor in young children's social development, and may be particularly beneficial to children at temperamental risk for poor social adjustment.