Children with negative competence beliefs often achieve below their potential in school. This randomized field experiment tested whether engaging in positive self-talk may benefit these children's mathematics performance. Participants (N = 212, Grades 4-6, M[subscript age] = 10.6) worked on the first half of a standardized mathematics test, engaged in effort self-talk ("I will do my very best!"), ability self-talk ("I am very good at this!"), or no self-talk, and worked on the second half of the test. Compared to both the conditions, effort self-talk benefited the performance of children holding negative competence beliefs: It severed the association between negative competence beliefs and poor performance. By internally asserting that they will deliver effort, children with negative competence beliefs can optimize their achievement in school.