This study examines the roles of cognitive flexibility and reading motivation in explaining the longitudinal link between teacher-student closeness and reading achievement. The investigation is motivated by the fact that cognitive flexibility and reading motivation have been shown to be correlates of teacher-student relationship and reading achievement, yet their mediating roles are less well understood. The current study uses a sample of 17,342 students (8463 females; mean age = 73.42 months) from the Early Childhood Longitudinal Study with different ethnic backgrounds. A declining trend of teacher-student closeness from kindergarten to Grade 2 was found. Teacher-student closeness at kindergarten was positively associated with reading achievement at Grade 4 and the effect was mediated by cognitive flexibility and reading motivation at Grade 3. Declining closeness from kindergarten to Grade 2 was not related to the other associations. Consistent with the extended attachment view, these findings highlight the importance of an early supportive teacher-student relationship in promoting flexibility in thinking and interest in reading. This enhances subsequent reading performance in the middle elementary school years. (author abstract)