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
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.